The cause of pain in the back of the knee can be a variety of things, including arthritis, sprained joint, mineral deficiency, overexertion or other issue. In many cases the pain may not go away on its own. The only way to be certain what's happening is to see a physician and let him or her diagnose it. Treating one possible cause could make the problem worse if it the real issue is something different. I really encourage you to see a medical professional and not try to self diagnose.