What Can Cause Rashes On The Inside Of The Arm?

This morning i woke up with a rash on the inside on my arm. Since then it's gotten bigger and spread to my forearm and wrist and my other arm, but not as bad. I've also been on birth control for about a year. I usually bleed every two weeks on it, but i haven't bled in two months, now i'm slightly spotting, when normally it's a heavy flow.