This is something that could turn out to be serious. You could have developed a DVT (deep vein thrombosis). Basically a blood clot. You will need to get this checked asap, it is nothing to fool around with.
My Leg Hurts Behind My Knee. Can It Be A Blood Clot?
Could be varicose blood vessels that have ruptured or burst
Hmm I really don't know google it or go 2 the doctor