If your 5-month-old baby has had red/irritated buttocks for 4-5 days and small white bumps have appeared today, it is recommended to consult a doctor. Common causes of diaper rash include prolonged contact with a soiled diaper, diarrhea, scented detergents, or irritating substances in certain diaper brands. To treat and prevent diaper rash, change diapers promptly, gently clean the skin with warm water, use alcohol-free disposable wipes, apply a fragrance-free zinc oxide or petroleum jelly ointment, avoid talcum powder, use superabsorbent diapers, let the baby's skin air out, ensure a proper diaper fit, and do not use corticosteroid creams without medical advice. If the rash persists for more than 4 days, comes with fever, blisters, or infected bumps, or spreads beyond diaper contact areas, seek medical attention.