Is it itchy? The peristomal skin is often prone to yeast infections because it's a nice moist breeding ground for opportunistic bacteria. You'll know it if you have one. It itches like crazy. Believe it or not that was the worst part of wearing a bag for me...you can't scratch it either.
If that's what it is, you can get a topical antifungal to apply to the area or if it's bad enough, go the systemic route and ask for fluconazole. Do you have an ostomy nurse to talk to/see to check it out?
Alison