Knee issue. I fix it in my sleep?

Okay, this is kinda a mystery. I am a 20 year old male, 5'10 250 pounds. Pretty overweight, I know. Anyway, the problem lies with my left knee. When my knee is normal, I can use it without any problems. When my knee is normal, the only thing I can notice is it's a bit weaker than my right. Here's where the problem lies. Sometimes, when I'm bending over, or moving my leg in an certain way while laying down or sitting, if feels if something pops out of place. The pain is in the back left part of my left knee. When this happens, I am completely unable to bare any weight whatsoever. I've been to the doctor while it was like this, x-rays show everything is normal, but obviously it isn't if I can't walk. The weirdest part of it all is when I feel something pop out of place, if I immediately lay on my right side, and hold my left leg to my chin for a while, it might go back to normal. The time I can't bare weight varies between a day to two weeks. I think sometimes I fix it in my sleep?? Help