Burning feeling in both knees?

i'm 16, and ever since i've been 12 or 13 i've have pains in both of my knees that would occasionally occur. I was told by a doctor that it was just growing pains. it slowed down & i would only get the pains a few times a year. but recently it came back. the pain is like both knees are on fire, and they feel heavy & its hard to walk on. i don't know if it is my joints or just from muscle use...any ideas?? i don't really want to go to the doctor because it's really hard to explain & they would probably just say its still growing pains. please help thanks:)