What do I do about my armpit rashes? Could they be yeast?

Armpit rash. You're on the right track. Your armpit rash is probably not yeast, which occurs mainly under the breast creases, abdominal creases, or genital area, but by any one of a dozen common skin fungus. It is most likely to be a fungal infection. It should respond to the same cream as used for yeast, e.g., otc Clotrimazole cream 1% should do fine.