Thumb pain?

OK, I feel really silly for asking this but both of my thumbs have been
both weak and sore. If I do not ride for a few days (out of town on
business), it goes away. It seems to me that there is something wrong
with the way that I am riding or with my bike fit. I checked the site
mentioned earlier (http://www.sheldonbrown.com/pain.html) but did not
see this mentioned. Any suggestions would be appreciated.