Clotting, or clumping of blood cells, is necessary to seal off damaged blood vessels and prevent excess bleeding. But when clots form inside healthy veins or fail to dissolve after the vein heals, a dangerous situation can arise. If the clot breaks off and flows to another site in the body, it's called an embolus (EHM-boh-luhs). Depending on where it lodges, an embolus can cause a heart attack or make it impossible to breathe. More commonly, however, a clot remains in place. Known as a thrombus (THRAWM-buhs), it can block blood flow, causing damage or tissue death in the affected area. Another possible complication of blood clots is phlebitis (fluh-BYE- tihs), a vein inflammation that usually appears in the legs. Phlebitis can result in sore, cord-like veins with swelling or irritation of nearby skin. You may feel throbbing or burning under the skin or a heavy sensation in the lower leg. When deep veins are affected, you might experience severe pain, swelling, or ulcers on the affected leg. Venous ulcers are open sores caused by the inability of leg veins to drain properly. They usually appear on the inside of the ankle or on the shin. Typically, the ulcers are accompanied by a swollen ankle; thick, leathery, discolored skin; itching; or burning.