I'm only 14 years old and i'm starting to feel very exausted after mowing the lawn for a very short peroid of time, and my muscles always hurt after i do anything. When i go up/down stairs fast i feel like i just ran a mile, even when i go up the stairs slowly, i feel like i just ran and start to feel throbbing/pain in my legs. I havn't felt the need to take any medication, what is going on?