Pain on right wrist from riding mtb

I have developped some pain from riding mtb for the last few months. It could be from holding the handle bar too tight, shifting too often, etc. The general location is where thumb meets wrist. Certain moves/angles give me a bad pain. My bike is not a famale specific design and I am guessing the right shifter is too Heavy/far for my small and delicate Hand. <img alt="wink.gif" class="bbcode_smiley" src="http://files.kickrunners.com/smilies/wink.gif"> Tendinitis is my suspect. Any suggestions?