What Causes Pain And Popping Sensation Of Knee?

I have a history of my knee popping out of place. That same knee tends to sometimes have a painful popping when standing up (I ve always sai it feels like air bubbles). As soon as the popping stops it s completely fine. Well now my knee feels like there is pressure in it and I m having a hard time bending. Painful pressure that I can almost feel through my leg.