What Causes Pain And Swelling Of Thighs?

I just noticed that the back and inner part of my right thigh is swollen. It first felt like a slightly pulled hamstring muscle a couple of days ago but then was a little alarmed when I could see and feel the swelling in the upper inside part and the back of my thigh. I am not in any real pain and i have full mobility and I don't recall any injury or over exertion. As a matter of fact i walked four rounds of golf the last four days. The urgent care facility closes at 9:00 so I plan to go tomorrow morning. Any help now is appreciated.