It is possible that the masks did cause the break-out. A lot of beauty products have stuff in them that will clog your pores. I would recommend only using products that specifically say "non-comedogenic" or "won't clog pores" ("oil free" isn't the same thing). It is also possible that you are allergic to the masks and that could be a rash, rather than pimples. In that case, try things labeled "hypoallergenic."
If the acne or rash doesn't go away in a week or two, go to your regular doctor or a dermatologist. I don't think it's anything serious, but you may need some medicated cream for the rash.