Carpal Tunnel can cause pain in the same spot. If you sleep with your wrist folded over or bent, that can cause the same pain in the same location as tendonitis. My left elbow started hurting a few weeks ago out of the blue. I had changed pillows at night that I prop my left arm on (side/stomach sleeper). Just enough difference to make my elbow hurt.
It's been amazing to me all the pain that has been relieved by having carpal tunnel surgery on just one hand so far. My forearms have hurt for years and my delts have been hurting for 6 months or more. Since I had my left hand done I pain no pain in either area on my left arm.