What's the cause of itchy blood clots on the thighs ?

Phlebitis: Superficial phlebitis is usually caused by local trauma to a vein. Superficial phlebitis may or not have a blood clot form to cause the pain and inflammation. In the legs, superficial phlebitis can be associated with varicose veins. Inactivity (blood pools in the veins and tends to clot if a person is inactive for a prolonged period of time); trauma, and blood clotting abnormalities.