I'm experiencing pain behind my left knee.feels like its giving in n i cud fall.Y is it happenin?wat shud i do?

I had a similar thing with my right knee. i thought it might be patella subluxation (your kneecap loose) but it ended up being that my hip was a bit loose so my knee felt the toll more. go to your doctor! i ended up having to go to physical therapy. it helped but as soon as you stop doing the exercises the problem happens again. hope this helps!