 [ Symptoms ] underarm rash during menstrual cycle every month usually a day or so before my period i develop a rash underneath my arms near but not in my armpit its usually a cluster of red dots that sometimes itch sweating tends to make it worse the itching i thought it was soap detergent deodarant or shaving cream changed each one at a time to test and see nothing changed the rash usually goes away within a few days but is very embarrassing [ Disease ] shellfish allergy typically does not go away there is extensive cross reactivity between all shellfish an allergist can help sort this out and can provide dietary recommendations if you are still shrimp allergic i would recommend that you have an epi pen in case of accidental ingestion