Why do my legs swell and itch while exercising?

I had knee surgery and MCL (hamstring implant) replacement back in March. I never had this problem before my surgery and I have noticed if I even stand for longer then 10 minutes ( just standing, not exercising) my calf gets bright red and swelled and very itchy. Today in physical therapy it got so bad I had to stop for the day. What could this be caused from? Like I said, I have never had this problem before, I dont have dry skin and I dont believe im allergic to anything... Any ideas?