What Is The Redness And Swelling In Knee?

i have a huge selling in my left knee which is very red and hot and now bruising is starting to appear and i cannot put any weight on it. i am 44yrs, 5ft 2 1/2 an weight aboy 8 st 5 ib. i had breast cancer last year on my left breast, had a lumptectomy followed bt radiotherapy. suffer from ulcerative colitis.