My 4.5 month old son recently discovered how to jam his thumb into his mouth while nursing. (Yep, while my nipple is still in there.) It can be slightly painful when he does it, so I try to gently remove it and just hold hands with him. I know he's discovering new things lately, but I was wondering if this might just be a sign he's got a full belly and wants to just comfort nurse at that point? Sometimes if I offer a plain thumb-free boob afterwards to him he just arches away from it and acts upset. Then I put him down and he goes to town on his thumb, fingers, entire fist, etc.