One knee is swollen. Slightly warmer than the other but not red. No injury. It has come me on gradually over about 3 months, initially only if I kneeled back on it. Now I can bend it but not all the way without severe pain. Some intermittent clicking/catching (sometimes painful). Going down the stairs is uncomfortable as well. If I had injured it, it would make sense so I'm confused. Any suggestions in the mean time?