How Can Back And Buttock Rashes Be Treated?

I am a foster parent of a four month old baby girl whose bio mom uses meth. it is suspected that she breastfed while using meth. the baby has a rash on her lower back and bottom that I am having trouble controlling. I keep it covered in desitin and it does nothing to help. Is this a side effect of being exposed to meth? Is it possible that the toxins are trying to get out of her body? Or do you think it is not related?