No panic ! It is quite common. It can be caused by the nappy itself in wet contact with baby's bottom, or even caused by a light digestive problem or a growing teeth.
First thing, be very gentle with your baby as it hurts ! Change the nappy frequently, wash the bottom with a soft washing product. After changing your baby, apply a thick layer of diaper rash ointment.
If irritation goes on, please consult your doctor.