I Am A 51 Year Young Woman. Very Active, I

I am a 51 year young woman. Very active, I dive, surf, boogie board,etc. I haven't fallen on my knee or anything but for the past two weeks my left knee has been killing me, it gets cold a lot and the pain goes up my thigh. What's going on with it? Otc meds are not helping nor is my heating pad. Help.