My DS had this a lot, burn from where his poo touched his skin. He has never showed any other signs of allergy though. Even if I cleaned him immediately (I mean IMMEDIATELY) following a poo he would get the burn and areas would sometimes bleed, metanium usually cleared it almost over night. Various foods would burn more than others, I was advised to watch out for red and orange foods. The problem ended when he started to poo in the potty, so none of it touched his skin. It was one of the reasons I offered a potty at 10 months, not expecting PTing over night, but getting those poos in the potty really saved us a lot of bum sores.
Watch out for thrush, more like diaper rash with pimples, little white dots. It can show in the mouth too. This won't clear with diaper rash cream like and needs prescribed meds for thrush.
