Noticing one leg suddenly becoming red, warm, and swollen is often more than just “standing too long” or a minor injury. Experts warn it can signal a serious health issue needing quick attention.

Deep Vein Thrombosis (DVT)
One major cause is deep vein thrombosis (DVT), when a clot forms in a deep vein, usually the calf or thigh. It leads to “pain, swelling, redness, and warmth,” and can become life-threatening if the clot travels to the lungs. Risk factors include long flights, surgery, pregnancy, smoking, obesity, and birth control pills.

Cellulitis (Skin Infection)
Cellulitis is a sudden bacterial infection making skin red, swollen, and tender. The leg may look shiny, and fever or chills may appear. Without antibiotics, it “can spread quickly and even become life-threatening.”

Other Causes
Swelling may also result from superficial thrombophlebitis (a clot in surface veins) or venous insufficiency, when vein valves fail, causing blood pooling and leg heaviness.

If symptoms appear suddenly, medical help is essential.