You need to rest or you will not recover. I'd see a specialist. How often were you "pushing yourself" in the gym. What were your recovery times? Your knees take a lot of stress, especially when lifting. Soccer is also a big killer for knees. Not to say its the same issue, but the back of my knee used to hurt if i tried to squat low (mainly in the winter when it was cold). Sucked trying to strap into my board or get up from the ground. With a lot of stretching and correct leg workout at the gym and taking Orange Triad (good multi and joint sup) I haven't had the problem come back as its gotten colder again. I can prove that taking a joint sup helped, but more towards strengthening my legs correctly. Just because you beat your ass up at the gym, doesn't mean your body can handle it.. Go see a doc man, especially if your knee hasn't changed in a week with rest... Could be multiple things.