The serious blood clot that you are referring to is called a deep venous thrombosis (or DVT for short), these clots are dangerous because of their potential to travel to the lung and cause severe respiratory and cardiac compromise (this is known as a pulmonary embolus, or PE). most DVTs are caused by a combination of three things:
-immobility (allows blood to pool in the venous system, which is a risk for clotting)
-hypercoaguability (inflammation or genetic predisposition to clotting)
-vessel damage (trauma, either direct or indirect (ie via inflammatory mediators or cancer)
The main clinical features of a DVT are unilateral leg swelling, pain, discoloration, and warmth. sometimes a physician will be able to feel the clot in the leg when pressing on the deep leg veins. In order to diagnose a DVT, a simple and painless ultrasound
test is performed and the clot can be visualized. Treatment involves prevented the spread of the clot or the formation of new clot with anti-clotting drugs ('blood thinners') like coumadin and heparin.
It is difficult to tell if the swelling and pain in your leg is due to the accident itself or due to a clot that may have developed as a result of the accident. If the symptoms persist and the leg remains tender and swollen, I would recommend you see a doctor
as soon as possible to have an evaluation in order to prevent the complications (ie PE as above).