In my case, a similar issue led to discovery of a pinched nerve in my neck. I don't know what made it go away, because I tried a few different things at once, including physical therapy. One of the things I did was to adjust my bikes for a more upright riding posture.

An unrelated but possibly pertinent issue: I also find that MTB style bars pound the hell out of my wrists, but that I can ride on swept bars with little or no discomfort.