The first thing to look for is a rash. Babies can be allergic to a variety of diapering items such as diapers, wipes, lotions, and even food that can cause a rash as their eliminations are caustic to their gentle skin. To find out what they are allergic to, you will need to try changing multiple items to see which is causing the issue. In the meantime, you can try cornstarch or a hypoallergenic baby powder along with lots of diaper-free time to dry out the rash and figure out what is causing the issue. Be sure to speak to your pediatrician about the suspected allergen.