Apart from the (sound) advice of giving it a rest for a while, and/or seeking medical advice, perhaps you should consider technical changes to posture, position of shoulder, elbow, wrist, etc. In classical guitar terms, this is not an awkward stretch, and shouldn't be causing the kind of problems you describe. (I don't mean to sound condescending, and for all I know your technique might be completely correct, but the ergonomics of how to play guitar is one area that has received a lot of attention on the classical side in the last twenty years or so, simply to avoid these kinds of issues.)