lt is most likely an allergy...there are many thing that anyone can be allergic to even food, water, cream, soap etc.

Kindly take her to a paediatrician for evaluation to know the cause of the itch. Also ensure proper hygiene.

There are 2 different types of nappy rashes in broad categories....those due to irritation from the urine and diaper...and those rashes that tend to occur in the nappy areas....It takes a Paediatric dermatologist to differentiate.

There are many causes of itching in children and antibiotics are not necessarily the Drugs for treatment of itching... It depends on the cause of the itching. Indeed drugs can also cause itching as side effects or allergic reaction
Kindly see a doctor for proper evaluation of the cause of the itching and appropriate treatment