I have a rash under each arm just when i think its better it returns along with this the creases on my mouth are cracked and sore what could this be?

Chelosis and gut : Chelosis is the cracking on the side of your lips. B vitamin deficiencies are a common cause. The rash could be caused by intestinal permeability that can lead to cell lining destruction that can lead to difficulty in absorbing b vitamins.

Vit issue, systemic: This can be from a variety of reasons. Most cracks in the mouth corners are from dryness and licking lips but can also be from a deficiency of vitamin b2, riboflavin. B2 serves as a coenzyme in helping the release of energy from the nutrients in food. A deficiency of b2 can cause the corners of your mouth to crack and can also lead to skin disorders such as the rash under your arms.