Severe pains in my side and no idea of the cause?

I have severe pains in my sides just under my ribcage whenever I sit down. I've been to the doctors and they said it was nothing to do with kidneys or anything like that. He said it would go away on it's own but it's been going on for absolutely ages now. I'd be able to deal with it, but the pain is excruciating and especially being at school still, and having to sit down for alot of the time, it's become a real problem. If anyone could tell me what's causing it, it would be much appreciated (: