What could be wrong with my knee?

I had knee surgery they did a lateral release 2 years ago and took out part of the broken off cartilage the knee pain went away for a while but recently something cracked\popped and now when I stand there is a lot of pain when the bone meets the bone. It's pain combined with strong pressure but it feels like there is nothing taking the impact. The pain is for the most part at the bottom of the patella aligned with the patellar tendon. When I put full extended swift on it it feels as if the back will give out or something is to tight what could this be