Hello dear, The cause of diarrhea in your child can be due to: 1. As a side effect of Amoxycilin, as it is known to cause loose stools. 2. Any infection in the Gastrointestinal tract which initiated in the form of vomiting, day before yesterday. The diaper rash can be attributed to the irritation of the skin due to continuous episodes of diarrhea. Try to maintain the hydration & electrolyte balance in your son by giving Oral Rehydration Solution (ORS) Pedialyte being an ORS needs to be fed..try feeding it little by little. If your child stops taking orally & severe dehydration sets in, then he will need IV fluids. So, kindly consult a Pediatrician & get a proper clinical examination done. Based upon that, appropriate management steps can be taken. Hope this helps. Please let me know if I need to clarify further. Wishing your son a Good Health. Take care.