My kneecap keeps slipping out of place, what should i do?

I am known for very week knees, and one of my kneecaps keeps playing up,,, it keeps slipping out of place ever so slightly and when it goes back to normal, it sort of grinds a little...? I'm not sure what that grinding could be but it's making my knee exceedingly painful, and when i've sat down for a while and i go to walk, i limp real bad. Does anyone have any similar symptoms, could you please get back to me soon, i have PE tomorrow and i really don't want to do it in this state, but just to prove i'm not exagerating and going mental i'd just like someone to explain to me the risks of it, and what the grinding may be... Thank you :)