OK now speaking from my own experience. Im an arm-picker myself. But i dnt find it bad or anything its just my preference. But can use my arm, wrist, and thumb for picking as well.

The problem thats troubling you is very natural. U must have practiced a lot, and so your fingers must have had a hangover. This usually happens to me if I keep on alternate picking fast for the whole day (about 11 hours). Then the next day I can hardly pick for 4 hours till my hand dies out. But then constant practice keeps building the endurance.

So keep practicing, and go with the day, and also make sure you push the limit of your picking a little, not too much, but a little everyday, and CONCENTRATE WHILE PICKING.

Sit in front of a mirror and practice, and look at your picking hand, IMO it will help you a lot....!!

:)