The constellation of symptoms that you are mentioning, in particular a red rash with an odor in this location, is most likely to be a fungal infection.
Fungal infections of the skin are relatively common, and yeast (a particular type of fungus) in particular, commonly cause infections of areas of skin that are moist, dark, and humid - places like in the groin and under the breasts.
Fortunately, this is not usually a serious problem, and it can often be cleared up pretty quickly with good skin hygiene and a topical anti fungal cream. I would, however, suggest that you go see your primary care doctor
about the issue prior to experimenting with treatments. This is because there are a few other types of skin rashes
that could occur and could be confused with fungal infections. These include eczema
, caused by excessive dryness of the skin, and contact dermatitis, an allergic reaction of the skin. The treatments for eczema and contact dermatitis are different from the treatments for fungal infections, which is why it is important to differentiate.
Make an appointment with your doctor at your earliest convenience. They will be able to confirm the correct diagnosis and suggest the proper treatment.