Lower back pains for days after any exercise involving torso?

I'm 14 years old and keep getting back pains for a few days after I exercise any muscle in my torso or arms, even with a very low weight or low repetitions. The pain soon goes away after a couple of days and i'm back to my normal self. My back usually hurts whenever i try to flex it or lie flat. I think that it may just be because i've grown considerably and the muscles in my lower back haven't quite developed fully, can anyone confirm this? (I'm currently 5ft11 which is quite tall for a boy of 14).