Why is my infant acting irritable?

Lots of answers. Lots and lots of potential answers -- lack of sleep, pain, hunger, change of routine, headache, mom's medicine penetrating into breastmilk and causing side effects, hair or thread wrapped around toe and strangulating it, ear infection, exposure to cigarette smoke, high stress levels in the house, heartburn in a baby with reflux, and the list goes on.
Many Possible Causes. An infant could be irritable simply because they are tired, hungry or need to be changed. However, irritability could also indicate pain, illness, acid reflux, constipation, etc. An irritable baby should be examined to rule out a serious cause.