depending on what is actually red it could be a UTI or anal gland issues those are the two easiest to treat and common problems. Of course it could be a whole host of other issues. Have you noticed diarrhea or straining to go to the bathroom?
Sounds like anal glands to me. A vet can take care of this issue.
Give her more fiber in her diet -- dogs usually love canned pumpkin (the regular pumpkin, not the pie filling). Keeps stool nice and firm and will probably cut out any future anal gland issues.