Why does my leg suddenly feel swollen, warm and sore?
One swollen, warm, sore leg can be a deep vein thrombosis (DVT). The 8 warning signs of a dangerous blood clot, when it has reached the lungs, who is most at risk, and how doctors check for it.
- Women's health
- Travel health
- Symptoms
Medically reviewed by Dr. William Aung, DocDocDash’s partner doctor network.
Last reviewed: November 2026.
The short answer: A dangerous blood clot most often starts in a deep vein of the leg, a condition called deep vein thrombosis (DVT). The 8 warning signs to know are: swelling in one leg, pain or tenderness in the calf or thigh, warm skin, red or darker skin, sudden shortness of breath, sharp chest pain that worsens when you breathe in, a fast heartbeat, and coughing up blood. The first four point to a clot in the leg. The last four can mean a clot has travelled to the lungs, which is an emergency. A Doppler ultrasound is the usual test to check the leg.
Get emergency help now if you have sudden breathlessness, chest pain, a racing heartbeat, coughing up blood, or face drooping, arm weakness or slurred speech. Call your local emergency number.
What are the warning signs of a blood clot in the leg?
A clot in the leg usually affects one side only, and that is the biggest clue. Both legs swelling after a long day is common; one leg changing on its own is not.
- Swelling in one leg. The calf, ankle or whole leg looks puffier than the other side, and shoes or socks feel tighter.
- Pain or tenderness. Often felt in the calf or thigh as a cramp, pulled muscle or deep ache that does not settle with rest or stretching.
- Warm skin. The sore area feels warmer to touch than the same spot on your other leg.
- Red or darker skin. The skin may look red on lighter skin, or purplish, darker or shiny on medium to darker skin.
About half of people with DVT have no clear symptoms at all, which is why risk matters as much as symptoms. If you notice even one of these signs after a long flight, surgery, pregnancy or a period of bed rest, ask a doctor to check it within the day.
What are the signs a blood clot has reached the lungs?
When part of a leg clot breaks off and travels to the lungs, it is called a pulmonary embolism (PE). This is the most dangerous stage, and it needs emergency care straight away.
- Sudden shortness of breath. It can come on at rest, not only when you are active.
- Sharp chest pain. Often worse when you breathe in deeply, cough or bend over.
- A fast or racing heartbeat. Sometimes with dizziness, light-headedness or fainting.
- Coughing up blood. Even a small streak of blood needs urgent attention.
PE can happen without any leg signs first. If these symptoms appear suddenly, do not wait to see whether they pass: call your local emergency number.
Can a blood clot cause a stroke or heart attack?
Yes. Clots that form in arteries, rather than veins, can block blood flow to the brain or heart. These clots are usually linked to high blood pressure, high cholesterol, diabetes, smoking or an irregular heartbeat called atrial fibrillation.
For a stroke, remember FAST: Face drooping, Arm weakness, Speech that is slurred or strange, Time to call emergency services. For a heart attack, watch for chest pressure or pain that may spread to the arm, jaw or back. Women are more likely than men to also feel breathless, sick, sweaty or unusually tired.
Both need emergency care. Regular heart and metabolic screening helps find the risk factors behind these clots long before an emergency. You can also read “Why “feeling healthy” isn’t the same as “being low-risk”?”.
Who is more likely to get a blood clot?
Blood clots form when blood slows down, when a vein is injured, or when the blood clots more easily than usual. Your risk rises when one or more of these apply:
- Pregnancy and the first weeks after birth, including after a caesarean
- Estrogen-containing birth control pills or hormone replacement therapy (HRT)
- Recent surgery, especially on the hips, knees, abdomen or pelvis
- Long journeys of 4 hours or more by plane, car or bus
- Bed rest or reduced movement, such as after an injury or illness
- Age over 60, being overweight, or smoking
- A personal or family history of blood clots, or an inherited clotting condition
- Cancer and some cancer treatments
For women living in Singapore, Australia or Japan who fly home often, or who travel abroad for surgery, several of these can line up at once. That is the moment to ask a doctor about your personal risk before you go, not after.
How do doctors check for a blood clot?
A doctor first asks about your symptoms and risk factors and examines your legs. Depending on what they find, they may order one or more of these tests:
| Test | What it checks | What to expect |
|---|---|---|
| D-dimer blood test | A substance released when a clot breaks down | One blood draw. A normal result makes a clot unlikely; a high result needs a scan to confirm |
| Doppler (duplex) ultrasound | Blood flow in the leg veins and whether a clot is blocking it | Painless gel-and-probe scan, about 20 to 30 minutes, no radiation |
| CT pulmonary angiogram (CTPA) | Clots in the blood vessels of the lungs | A CT scan with contrast dye, used when PE is suspected |
| Clotting blood tests | Whether your blood clots more easily than usual | Sometimes advised after an unexplained clot or with a strong family history |
The Doppler ultrasound is the main test for a suspected clot in the leg. It is quick, widely available, and gives a clear answer the same day at most hospitals.
How can you lower your risk on long flights and after surgery?
Simple habits can lower your risk, especially on flights longer than 4 hours.
- Move often. Walk the aisle every hour or two when it is safe, and flex your ankles and calves while seated.
- Drink water and go easy on alcohol, which can leave you dehydrated.
- Choose an aisle seat so it is easier to get up.
- Ask about compression stockings if you have any risk factors. A doctor or pharmacist can advise on the right fit.
If you are travelling for surgery, timing matters. Ask your surgeon how soon you can safely fly home, whether you need blood-thinning injections or stockings, and what signs to watch for in the weeks after. A good care plan covers the journey home, not only the operation.
Frequently asked questions
How do I know if leg pain is a clot or just a pulled muscle? You often cannot tell by feel alone. Pain that comes with swelling, warmth or skin colour change in one leg, or that appears after travel, surgery or bed rest, should be checked with an ultrasound.
How is a blood clot treated? Most clots are treated with blood-thinning medicine, usually for at least three months. This stops the clot from growing and gives your body time to break it down.
Is it safe to fly after a blood clot? Many people can fly once treatment is under way, but timing depends on the clot and your health. Always check with your doctor before booking.
A simple rule to carry forward
One leg, one change: swelling, warmth or pain on just one side deserves a same-day check. And sudden breathlessness or chest pain never waits.
Ready to take the next step? If you have risk factors or are planning surgery abroad, DocDocDash helps you compare vascular ultrasound and pre-travel health checks at trusted partner hospitals. Chat with our team in the app, and book.
Sources:
- Centers for Disease Control and Prevention, Prevent Blood Clots (DVT and PE signs and symptoms)
- Centers for Disease Control and Prevention, Blood Clots: A Serious but Preventable Medical Condition (about half of people with DVT have no symptoms; PE can occur without DVT symptoms)
- National Heart, Lung, and Blood Institute, Venous Thromboembolism: Diagnosis and Treatment