i had almost the same thing when i started shooting back when i was around 14
it was because my shoulders had not yet developed enough muscle to hold a bow out to the side straight armed. what you need to do is practice witha 10 pound weight holding it in front of you for about 3-5 minutes and keep it up going longer if you can. also make DAMN sure you stretch out yoru shoulders when you shoot.. most guys dont do stretches for shooting and as a physical therapist, i can tell you that no matter what activity you do you should always STRETCH those muscles out before hand.

good luck