You most likely hit your arm due to a cocked wrist and poor arm alignment when shooting. A very common error. When the bow is properly gripped you will not hit you arm anymore. The best way I can try to explain this is that you cock your wrist back when holding then grip forcing you arm forward where it gets hit with the bow string. Try holding you elbow slightly bent and keep you wrist from bending backwards as you draw and hold. This should correct the problem. Hope this helps, good luck!
I see your playing with your new toy.[hihi] That'll happen. I don't shoot year round and it almost always happens to me the first time I practice in the summer. Just a subtle reminder to watch my form. I correct it and it doesn't happen again ('til next season at least)...lol