Or could I have developed a mild allergy to it?

never bothered me... My question is, why are you using apple cider vinegar on your skin in the first place? No.

You probably have an allergy. I have used apple cider vinegar on my skin to exfoliate. It does turn my skin red for a while. I only leave it on a short while and then rinse it off. It's a cheap version of an acid peel. I only do this once a week and follow with a good moisturizer. After the redness goes away, my skin is softer.

I have also heard of it being used for certain rashes and as a hair rinse (diluted with water).

It could be you are using it too often and not giving your skin a long enough recovery time.

If you are taking it orally and having a response, perhaps you are sensitive to it.

Hope this helps. I personally find apple cider vinegar soothing to a sunburn and I use it on my legs for charley horses but it's possible that you've become allergic to it. You have an allergy to it. I'm the same with avocados. *sad sigh*