It sounds like stool is getting under your wafer and irritating the area around your stoma - this was happening to me until I found the system that works best.
I make sure my skin is clean and very dry, then I apply stoma powder where there is the redness. Dab off any excess powder to make sure your adhesive sticks correctly. Then apply a barrier (there are sting free pastes or a cohesive seal - like the Eakin or Adapt. Coloplast also has a Brava line with seals). Call manufacturers for as many samples as you can get - I called Coloplast, ConvaTec and Hollister as soon as I was discharged from the hospital and got a ton of supplies.
On top of the barrier ring I use a light convex flange - my stoma likes to shrink down and hide. Is your stoma flush to the skin, or does it stick out a bit? That will also make a difference. After two changes using the Eakin ring and convex flange all of my skin issues have cleared up. I can go 5 or 6 days between changes.
Your skin could also be sensitive because of your chemotherapy - do you have a wound/ostomy nurse you could contact for help?
I hope you can find something to help