What Is The Treatment For Rashes Under The Left Armpit?

I have had a rash under my right arm pit for almost a year now. It is now shaped and raised,almost patchy looking, but the itch is sometimes so intense it gets raw and burns. It is near the outside of my armpit. I think it started from different deoderants I was trying.. I haven't worn deoderant for months, but use baby powder now. I was using neosporin, but that didn't seem to work for long. I don't know what to do!! Thank you Dana