My mom had a rash under her arms that looked like spider bites what is it?

RASH IN ARMPITS: Without seeing the rash, it's difficult to make an accurate diagnosis. This sounds like a rash due to either a new sweater or blouse, or an underarm deodorant or a new soap, or new laundry detergent. Warm water compresses, a mild cleanser like cetaphil, and an over-the-counter Hydrocortisone cream, twice daily might giver some relief. If no better, have her examined by a dermatologist.

Unsure: This rash would likely need to be examined and more history obtained to be able to provide you with useful input about the diagnosis or possible treatment.