A rash can have a lot of causes, including an allergic reaction. She might be having an allergic reaction to fleas, grass or another irritant, a staff infection or simple irritants. This time of the year, it's often caused by bug bites.
I suggest that you bathe her well with a good shampoo. Then dry her well. See if this helps with the redness. If it's a contact allergy, it will give her some relief.
You should also give your vet a call and see if oral benadryl would be a good idea to combat any allergic reaction that she might be having. My vet recommends 1 mg per lb of dog given every 8-12 hours.
If this does not go away on its own and a vet visit will be needed.
Let me know if you need more help!
