I have sudden onset pain in the muscles on the underside of my upper right arm. It was periodic, only occurring when I slept with my arm in a certain position. Over two months that pain spread further along my right arm and began occurring during the day at times. Now it occurs frequently and I have difficulty with certain movements such as reaching back or reaching up with that arm. The pain is bad then. This week the same thing started happening to my left arm. There has been no injury or trauma to either arm. It began originally simply from laying on my right side with my arm placed under my pillow while I slept - a position I ve slept in for years. The first episode was sharp, painful and very sudden, no soreness leading up to it. What could cause this sudden onset of muscle pain, increasing in intensity and spreading along my forearm, and now starting to occur in my other arm?