My advice would be to take a break if you can, at least from drawing. If you really have to, make sure you are drawing from your shoulder instead of only moving your wrist. Pain is your brain telling you to stop doing the thing, and you have to listen to your body. the same repetitive movement is the problem, overworking it.
"taking a break" could mean a day, a week, or even a few minutes in between drawing.
I do go throught the same things, Usually happens because I start using my wrist, and after a few hours it starts to hurt. I thought that I just had to do hand exercices before drawing to just get used to it, but no ammout of hand streching is gonna solve it (it does helps, but not in the long term).