My dog keeps biting her but an she has a red rash on it but she also chews her feet too

Not sure about the butt part but the feet part could be allergies. My dog has allergies and we have to wash his feet daily as that is the area that gets it the worst.
How old is your dog? Have you changed food lately? Is she otherwise healthy and up to date on vet visits?
The more info you give the better and more accurate the advice can be.

Allergies, fleas, anal glands, would be my best guesses here.
If her anal glands are full and need expressing she may chew or lick at her butt, you may also see her scooting across the floor.
Fleas will make her chew also, rub her fur the wrong way on her rump and look closely for any fleas or flee dirt (tiny specks of black that will turn reddish-orange on a wet paper towel)
Look at her belly and under her front legs, the warm spots that fleas like.
Allergies will cause her to be really itchy and chew at her feet or anywhere else that is bothering her.
In any case if she chewing to the point of being red, she could work up a nice hotspot, causing pain, irritation and infection.
Fleas can be fairly easily treated with topical products, but don't go for the cheapest thing you can find, they don't always work well.
Stick with a trusted product like Frontline, Advantage and the like.
Your vet can guide you here.
If she is suffering from allergies or needs her anals expressed you'll need to see a vet to work on resolving the issue.
If she chews herself into a skin infection you will need to consult a vet to prevent it worsening.