Shoulder Cramping When Playing

Has anyone else ever routinely experienced cramping in his or her right shoulder when playing? That's my strumming arm, of course,and I can't figure out what's causing it.

I keep repositioning my dulcimer on my lap to see if it helps -- sliding it down farther to the right, etc. -- but nothing seems to make it better. It always starts doing this after one or two songs, darn it. :(

Laurel