Ditto the food allergy suggestion. I have a toddler and baby both with food allergies, and their ped always looks for the ring around the anus to see if they are currently reacting to something. The ring can also indicate any digestional upset caused by other factors such as thrush or a tummy bug.
Wife to my supportive DH
and SAHM to DD Bug (06/10) and DS Buddy (07/12)