another weird nursing habit

My baby will be 5 months soon. He has been sticking his finger in the side
of his mouth while he's nursing. He rolls his finger in there. It's so weird
and it pinches my nipple. Plus he doesn't get a good suction and he slurps
milk. I don't know if his gums bother him or he just likes the way it feels
to suck his finger and nipple together. I hold his hand but he gets it away
and puts it there real fast.
Kris