My son has a bad case of diaper rash and cries when we change him.
Diaper rash is very common in babies.
Even the most diligent of parents will not be able to keep irritation of the diaper area at bay 100 percent of the time. This is usually caused from a combination of the moisture under the diaper from the skin, the urine, and the stool. This is the same yeast that causes thrush. This is hard to do especially in infants. Try changing the diaper more often than you are used to.