The first thing I would try is to change the brand of mask you are using. Try latex-free and/or hypoallergenic. I’ve heard from hygienists who have the same problem liking Cranberry S3, 3M ESPE, or Alpha Protech. Don’t be afraid to try many different brands. Make sure to never put the mask under your chin and change your mask often because moisture trapped by the mask can irritate your skin. Also, not wearing makeup helps some hygienists. If nothing helps, I wouldn’t hesitate to see a dermatologist. Stress and hormones can cause acne and it may help to rule those things out.