My mucles are sore; it's mostly in legs and arms. Is it because I'm growing and my mucles are stretching, or is it just muscle joints or something else?
It is called growing pains. It comes from the bones lengthening and the muscles not keeping up with the changes as well. Reaching further means new muscle tissue is being produced that hasn't been completely coordinated with the muscle already present. The new muscle cells protest when they first get used, just like your muscles are sore when you exercise after not doing much for a while.
How long do growing pains last?
For as long as you are growing. They are worse during stage 3 when you are growing rapidly. Stage 3 generally lasts about two years.