Unexplained hip pain?

Im a 16 year old girl, and i have really bad hip pain, when i walk my right hip mostly and sometimes my left makes kinda like a popping cracking sound and is very uncomfortable, and when i move certain way (not strenuous) ,it makes very loud cracking like you would crack your knuckles but louder and its very painful im normally pretty active but if i walk alot the next day both my hips ache to an almost unbearable level, iv gone to the doctor and got xrays but they said everything was normal so what can cause this?