What Causes Severe Pain In The Knee?

Hi, im 15 years old and have had a bad pain in my knee for about 1-2years. I've been to the doctors lots of and they said was just growing pains, then last year i was in hosptial having test for joint infection then in the end told it was a sprain. I still have the pain and my knee gives away alot, I cant join in with P.E in my school alot and have to limp otherwise i have a shooting up my knee if I walk properly. I havnt fallen or hurt it in anyway to cause this problem, any idea what is wrong with my knee??