Sounds like you have one bad knee....Shorebreak wrote:I am only 20 years old so I know I am no where near close to having bad knees.
It is commonly referred to as a Knee Effusion. The most common cause is arthritis. My disclaimer, I am not a MD. The problem when I saw it in the clinic usually resulted from overuse or a Meniscus tear. In your case since it is random there is a good chance that you have minor ligament injury that is undiagnosed. I have had this happen before and was diagnosed with a Meniscus tear that was not bad enough for surgery. The Meniscus acts like a shock absorbing type of cartilage in the knee. The first thing I noticed was a "clicking" in my knee when I would flex and extend my lower leg. For the most part I was pain free until it started to swell up. After about 8 hours and alot of motrin it would feel good enough to walk on. The symptoms were random and would pop up with no warning. I would be pain free again in about 24 hours.
After seeing a sports med doc I was sent to PT and given exercises to strengthen the muscles in my knee and proper stretches to do before running and have not had a problem in years. Go get yourself checked out. Whatever the problem is it will not resolve itself.