You say you were practicing "on and off". So you are obviously taking breaks during the long practice session. This is important IMO.
I dont know what stretches you use for martial arts, but here are some for the bass. Spend two or three minutes doing these, then pick up the bass and ease in gently and slowly with some chromatic exercises high on the neck. Do the stretches before and after each practice session. http://www.youtube.com/watch?v=7_0aVxFxBa0 http://www.youtube.com/watch?v=C_FZichHw1w
Make sure you are well hydrated and are not playing on an empty stomach.
You may also need to have another look at your technique.
For now, I'd rest the hand for a day or so. When you start back, take regular rest periods if playing over a long time.
Hope this helps.