I Have Unexplained Bruising On The Back Of My Leg,

I have unexplained bruising on the back of my leg, behind the knee and the back of the calf. The bruising covers a large area, but I have no recollection of doing anything that would have caused it. I initially noticed it one morning while I was shaving my leg. Then another large bruise appeared lower on the calf a number of days later. My hamstring has been extremely tight and my leg feels swollen.