Why do the back and inner of my thighs hurt?

The back of my thighs and the inner part of my thighs really hurt... I'm 15 a healthy weight I eat and exercise well, I haven't done anything strenuous lately to trigger this, but my thighs hurt all the time! What are some general causes of thigh pain? It's more of an ache I guess but collapsing in bed doesnt ease it and nor does 'walking it off' btw I can't take pils either.