Jaz, if i recall you're only 14, maybe your body is just going through a growing phase, bones are stretching etc etc, also are you pushing big gears?
Many years ago when i was a junior, the top 2 spockets off my cassette, were removed to save me pushing to big a gear and damaging my knees, legs as i was still growing.
Not sure if they still do this, but maybe don't force the big gears for a while, sit in small front ring whilst out see if that helps.