Deep Vein Thrombosis (DVT) occurs when a blood clot forms in one of the
deep veins of your body, usually in your legs.

These symptoms of a blood clot may feel similar to a pulled muscle or a “Charlie horse,” but may differ in that the leg may be swollen, slightly discolored, and warm. Contact your doctor if you have these symptoms, because you may need treatment right away.
Clots can break off from a DVT and travel to the lung, causing a pulmonary embolism (PE), which can be fatal
Call an ambulance or 911 immediately for treatment in the ER
350,000—600,000 people in the United States develop blood clots every year. About 100,000 people in the U.S. die each year from blood clots, which means that about 1 of 3 may die.
Reference:
The Surgeon General’s Call to Action
to Prevent Deep Vein Thrombosis and Pulmonary Embolism, US Dept of
Health and Human Services, 2008.