I Have Knee Pain When I Lay Flat In Bed,my Knee Locks And Hurts Like Hell,when I Bend Down It Goes Out,Any Suggestions?

This sounds exactly like what I have... I don't know what it is either. I wanted to ask this person if they've found anything out about it.I'm only 19 so I really don't know if this is serious or not. Please help because no doctor can tell me what my problem is.