stop using whatever deoderant you are. you are having an reaction to it. even if you used it all your life you can react to it like that. it just happens.
take a detox bath every night.
1 cup of Sea Salt in bath
soak for 30 minutes
scrub off and rinse
pat some aloe on your pits and some baby powder or cornstarch at night.
if you can get away with it during the day until the rash heals that would be ideal.
i am sensitive to deoderants like this sometimes. it totally sucks to armpit rash.