Why does my 13 month old develop some pimples on the neck and on the stomach?

Infant Rash. It is essentially impossible to assess a rash without performing a history and physical. Acne is, however, not uncommon at that age. If you feel like the rash changes in a significant way, or he/she develops other symptoms that are concerning to you (including, but not limited by fever, decreased oral intake, decreased urination, lethargy, inconsolability), please have someone assess him/her.