Hi . I hope you get some input from others in the community, but I'm guessing that's your immune system responding with histamines to ward off any potential infection just like it would with burns. We're not medical experts though, so I would send a quick note to your dermatologist to get assurances that the reaction is normal. I'm glad you're done with the Fluoracil. That must be a relief. - Lori (Team Member)