My 5 month old frequently hits himself in the mouth, or sometimes his stomach, but he's never crying when he does it. is there a reason why?

Reading babies minds: If only i could read babies minds my job as pediatrician would be so much easier! obviously we are not absolutely certain as to why some babies behave as they do; however, by acknowledging that he is not crying suggests this does not seem to hurt. Perhaps your son is still attempting to learn how to use his arm muscles by attempting all sorts of gyrations that randomly hit various body parts.

Muscle control: At 5 months, most babies don't have good control of their extremities. It is likely random movement. If it seems more deliberate, talk to your pediatrician.