Knee pain, got xray, doctor said no fracture, what could it be?

Sometimes when i walk I feel like my knee gets kind of dislocated? And I stand there trying to pop it after it makes that pop noise, no pain it feels a lot better. At times I have to stand there to stretch it out because it can hurt. Doctor said it was nothing but its obviously not true, xrays showed it was nothing as well, what is it?