for the past 2 months i've had bad pain in my left knee.

First there was terrible pain (out of the blue) and then after nearly 5 hours, a loud pop followed by relief. Since then, the pain has remained in the back of my left knee. It hurts to straighten or bend, and to put any weight on at all after it has been bent or straightened for any length of time. The pain is such that I often avoid it's use, including measures so extreme as hopping on the other foot. There is no redness, and I have no fever. Possibly connected to an infection I believe I have on the same shin of the fatty tissues under the skin. (resulting in knot-like red spots that spread. online it said it can cause joint pain as well)