In the last week, my right knee has popped/slipped out of place twice, and my left knee once. I have never had any trouble with my knees, and I am young and healthy. All 3 have been when I have sat down- when I fold my legs under to sit on them my knee slips out. It's really painful, but not unbearable, and my knee/lower leg will pop back in if I slowly stretch my leg out. Does anybody know what this is, and do I need to see or doctor?