peculiarities of form

Recently, whenever I try to write (or type, sometimes) or in other ways use a pen or pencil, my hand starts to hurt. Thankfully, it's not in the wrist, but seems to be at the base of the thumb. I don't see anything, and it doesn't feel different from my left hand (although the indent is a little more pronounced) and it doesn't always hurt if I press on it - usually it does, though, provided I find the right place - so I'm not sure if I should do anything about it. Hopefully it's just a bruise, but I don't know how that could have happened either. It's a little upsetting.