@oboogie said it above and I would agree. It is probably an adhesive allergy.
My tattoo on my leg I just got a week and a half ago was my first experience with saniderm/tegaderm like products. Long story short I have the red spots or rash now and it is a pain.
You are probably allergic to bandaids as well as they apparently use the same adhesive. I knew my body was mildly allergic to bandaid adhesive but thought saniderm would use another kind of adhesive since it is applied over wounds with the adhesive touching the damaged areas. But I guess not.
And to your point, I have the reaction on part of the skin where saniderm was applied but not all of area. So maybe different areas of the same persons skin react differently.
Just for reference, I can wear a bandaid for a day maybe but much more and the rash shows up. My Mom and Brother can only have adhesive on their skin for an hour or so before the reaction. The reaction can be very severe with blisters etc.