What Causes The Urge To Pop Knee After Bending It?

My knee has been killing me and it feels like it needs to pop, which it usually never has popped before. I just got don t running and I can t do the simplest things like bending over or squating. It doesn t really feel tight it just as soon as I bend over or something it feels like I need to pop my knee cap back into place. I know my knee cap isn t misplaced but I dont know what could be causing the trouble