Misfits has given good advise because it really does sound like an impacted anal gland issue.
Anal gland impactions are often related to skin allergies or atopy.
Please take him to the vet for a rectal exam and anal gland expression. While there that vet could investigate further if the anal glands turn out to be normal.
I'm not the Vet, but will answer this.....Since you are in Florida, a flea allergy will cause the reaction your describing......Fleas will usually bite the rear/rump. One bite can trigger the reaction although you may not see a flea....If you have no fleas, then she needs her Anal Glands/Sacs expressed....For many reasons, Anal Glands do not empty on their own as they are intended....Once they get impacted, they are very painful!
Your Vet or groomer will be able to do this for you.......IMO, After expression, they are sore for a couple of days.....Good luck