If your baby has red bumps on their buttocks, it could be due to irritation from urine and feces, sensitive skin, diarrhea, fungal infections, or tight, plastic diapers. To help alleviate the irritation, you can change diapers frequently, wash the area with water and mild soap, apply a protective cream, let your baby's skin air out when possible, and use highly absorbent or bamboo, hemp, or microfiber diapers. If the rash persists for more than four days, your baby has a fever, blisters or boils appear, or the rash spreads beyond the diaper area, it may be a good idea to consult a doctor for further evaluation and treatment.