Sounds like chondropathy to me, I'm suffering from this condition for quite a while now.
The culprit is a piece of tissue which has softened up, allowing more friction and pressure which causes severe pain and damage to the joint. It can be diagnosed using x-ray images taken with and without pressure on the knee by comparing angles and distances between the parts of the joint. I've been told to strengthen my quads, take green mussel extract and enjoyed 10 sessions of laser therapy. The pain disappeared for about 2 years, since then I have severe pain outbreaks every couple months lasting for 2-6 months at a time. About to visit a highly renowned knee specialist in June, to find out what's next.
With all this being said I recommend rest, green mussel extract, and in theory collagen and other joint supplements should help you in the long run. Also...get an appointment with a specialist.