When that happens to me I stop wearing makeup (no matter how hard it is to go out w/o it) b/c I don't want to irritate it more. That way you can use a super-mild cleanser. My skin freaked out recently when I tried benzoyl peroxide and I picked up Dermalogica's sensitive skin kit...mild cleanser, mild, hydrating, spray-on toner, and non-irritating moisturizer. It calmed my red, bumpy, inflamed skin down w/in a couple hours. Didn't think of taking Benadryl, but that's a great idea!
If your skin isn't feeling irritated any more, you can start trying to exfoliate gently (like with a washcloth). Just be really gentle with it for the next few days to a week -- I find that works best.