What Causes Rashes Around Neck, Under Arms And Breast?

I keep getting a rash around my neck and occasionally under my arms and breasts when I am on my period. It seemed it was due to taking Tylenol, so I eliminated acetaminophin (and it seemed to work) and am only taking Ibuprofen. Now 2 days into my period, I have the rash on my neck again. Any suggestions of what this could be? This started about a year ago and only happens when I'm on my period.