What Causes Blood Clots And Swelling Of Knee?

hi i have had this kind of cramping pain in my left leg for a few days and it was worse yesterday, but today it seems to of eased off, it goes from my bum down to my knee, the knee looked swollen yesterday and now and again it would go purple and when i touched it i would get red dots for about 30 secs or so then they would go, im just worried as blood clots are in my family im only 21 so i dont think i could get them could someone help me please