If your baby is showing signs of a rash while taking amoxicillin, it is important to stop the antibiotic and consult a doctor. The doctor will evaluate if the antibiotic is the cause of the rash and may suggest a different antibiotic if needed. In the meantime, you can contact your pharmacist for the correct dosage of Benadryl for your child based on their weight. It is important to follow up closely with both the doctor and pharmacist to ensure your child's well-being.