I agree with Velocipedio's idea of the angle of the the hoods. That could easily put stress on the wrists.
Another idea would be the back brake.
Did you have to grab the brake hard a few times? If that is the case, you may have strained a muscle.
Did you have to keep your hand over the lever more than usual? If that is the case, you may have stretched a muscle.
Either of my ideas should go away after a few hours or a night's rest. Velocipedio's idea will most likely take longer to fully heal.