Extreme pain under the knee cap?

I have had a bad left knee for a while in my left but now its starting in my right leg, There is a bone under the knee cap and it hurts when I touch it, I can't run on my leg properly and it hurts when I get up from crouching or sitting. also If i Sit for long periods of time my leg goes stiff. I am 14 and Im not massively active or mad sporty but I do run around. I drink alot of sugary drinks. Ermm, has started In my right leg as well, same simptons but not as painful