He may just be having a bad day, like all creatures have. You may have spooked him, you may have had something that smelled "yummy" to him whever ever he bit you, etc. Don't fret on this, it happens to all of us. Give him some time and pick him up again. Just don't stand there and hesitate when about to pick him up, just grab him and if he bites, he bites... if not then great.