What could cause red, blotchy spots under your breasts and on your stomach?

Eczema or infection: Could be eczema or folliculitis (breakouts similar to acne) or a type of yeast infection. Eczema can be treated with topical steroid creams. Folliculitis is treated with topical and/or oral antibiotics. Candida intertrigo (yeast infection) can be treated with topical antifungal creams. You need to see your doctor for proper diagnosis and treatment.

Difficult to answer: "red, blotchy spots" could mean a number of things from simple skin irritation to fungal or bacterial infection along with a number of other things. Having a doctor take a look will be your best option.