This happens to me too, so your not the only one (and I'm 13!) what i say is, don't run for long periods of time, and if you have too, just jog for a while. If you have to walk, well, it's fine, as long as you don't just spontaneously do something else. Also, yah, rest after you do something that includes physical labor.
And my best advise (because that advice probably stank) is, at night, do some imaginary bycicalling in bed. Just pretend you have a zero gravity bike, and you want to keep pedaling upside down. it will hurt for a while, but then, it will help you feel better, during the day, and at night.