Hmm, sounds like what I got a while back on my knee. Kept me from doing squats for the longest time, even after it healed up. Didn't get it from the gym though. County lockup is also a common place for this I guess.

Looks kinda like a spider bite, right? A really bad spider bite. Nasty open sore with... well, I won't describe it any more. Let's just say I had a quarter sized hole in my knee, and could almost
see the knee cap it was so deep.