You can put her in the tub with warm water and baking soda a couple times a day, change her diapers very often, use your diaper rash
ointment every time and try to keep her as dry as possible. Also, try to leave the diaper
a little loose so some air can get in. Simply try to keep her dry and give it a couple days.
When it comes to the diarrhea, if she is on milk based formula and is vomiting as well she may need a soy based formula, my daughter did.