Have you tried different deodorants? Sometimes you can have an allergy to some types. I was getting breakouts in sensitive areas of my body like that and we changed to Tide-Free detergent (dye-free, perfume-free, hypo-allergenic) and I haven't had any problems since. My doctor also told me to only use Dove for sensitive skin. Because it's specifically in your arm pit, maybe it could be something as simple as that--or even the shaving cream you use on your arm pits. Good luck!