I haven't experienced much in the way of pain at the top of the forearm that I can recall, but certainly pain in the wrist, the area from the meaty part of the thumb extending down through the wrist and into the bottom of the forearm. If you are just starting this is normal as you stretch and strengthen things.

I forget where it is but in Lisa's fundamentals she goes over how to hold a guitar and she does talk about arm position, that may help you.