Hi welcome to Health care magic forum. We can't come to a conclusion at once. to have a clear idea about it, you better stop the cerelac and find the difference, if still baby is continuing with rash after 2 days it is confirmed that it is not due to cerelac. I suppose it could be a nappy rash or some thing of that sort. Also observe the reaction of the soap, powder, etc by stop and observe one after another. I advise you to consult a pediatrition for diagnosis and treatment.
Besides i advise some diet recommendations for mother, as avoid taking brinjal, fish, and sea foods, and some thing else to which baby is allergic.
Take more of green leafy vegetables, pulses, sprouts, milk, egg, meat, fruits, juices, and dry fruits, if breast feeding. Wishing for good health of the baby, and thanks for calling.