Another source of irritation could be deodorants and antiperspirants that contain ingredients that aggravate the underarm skin. You might not even be aware that these specific ingredients are irritating your underarm skin (I call them the silent irritators). When selecting a deodorant, try avoiding ingredients like baking soda, high levels of fragrance, or solvent alcohols like isopropyl alcohol.
Baking soda is an alkaline substance, and it can alter the pH levels of your skin, causing your skin to change color. Similar to how high levels of fragrance can irritate the face, it can also irritate your underarms, especially due to its delicate area. If the scent is strong when you open the lid of a deodorant, you know it is more than 1%. When it comes to scent in deodorant, I know consumers like it but I never put it in higher than .05% as I feel that you still have that lovely sensory experience without the irritation in the underarm region.