D.B.
Heat rash, yeast, UTI, food allergy. Increase fluids even if it's through fluid-rich foods. Decrease dairy for the time being just to see. Stop the baby body wash - it's not necessary even though you don't think it's the cause. You can try a little cranberry juice or cranberries in her diet. Urine should be clear if she's sufficiently hydrated and infection-free. No powder - increases yeast infections. This sort of thing is also very common in the summer.