What Causes Pain In The Back Of The Knees?

I have mild to heavy pain in my growing along with in the back of my knees. this happens about every six months and lasts for about a week. Sometimes the pain is sharp and can double me over and sometimes its like nothing is wrong. It sorta feels like everything is swollen but looks normal. I am a 34 male and this has been going on for at least five years. I have seen about 5 different dr s and they all look at me like i am crazy. Please help