Son (16 months) has a diaper rash with blisters?

In 2 spots, bright red areas with small/medium 'blisters' or raised bumps that appear to be fluid filled- one in the front, one in the back. Did not have them yesterday, has none anywhere else on his body. Usually wears cloth diapers, was in disposables today all day. We always use this brand and have never had any problems- what could it be? Can't think of any kiddie illnesses that present this way, can't imagine what the heck it is!