Suggest Treatment For A Rash Under The Armpit

I have a rash under my arms. My sister was diagnosed with celiac so I went gluten-free to support her at Thanksgiving. It seemed to help some mild digestive issues of my own, so I stayed with it. Now I wonder if the rash is related to accidental gluten ingestion. I doubt it is contact dermatitis because I have not changed any products, only use natural, non-chemical products. The rash goes away with OTC cortisone cream.