My dog was groomed a day ago and is growling at her butt.

Are you sure it is clipper rash or did the groomer try to empty his anal glands?

Clipper rash is caused by a combination of the heat of the clippers, plus pushing hard into the skin.
My white poodle gets it terribly when he is clipped with anything higher than a #15 blade so I got a cream from my vets that is both an antiseptic and an anesthetic, so it prevents infection, and numbs the area so it's not so itchy and painful.

Next option is the anal glands. A lot of groomers will express the glands whether they need to be or not and it an hurt the dog.
Anal glands are basically a scent gland situated near the opening of the anus. Every time a dog defecates a little of the fluid is expressed to mark their territory. If a dog is on an incorrect diet and their stools are too soft the fluid is not released and there can be a build up, eventually leading to infection. Hence the need to squeeze and express them.

This really has nothing to do with this post.. But.. If your dog happens to have anal gland issues like my moms past two dogs, then feed them pumpkin every so often. The pumpkin acts like a big ol laxative and creats a huge bowel movement excreting the fluids naturally.

Works like a charm ;)

Has he been checked for worms?