Thanks for the details.
This seems like a fungal rash. Especially because of the area you mentioned. Skin folds, underside of the belly or the breast are usually warm, moist areas that provide a good atmosphere for fungi to grow in.
A lack of silvery appearance, discharge and drainage or pus rules out rash of bacterial and autoimmune etiologies.
Although visual examination is the best evidence to a proper diagnosis but in this case the clinical features are very clear.
I would advise you apply Lamisil cream (anti fungal) thrice daily. Keep the area dry and clean.
Take a warm bath, clean the area with a soft cloth, do not rub or dry it with a rough towel. Dab it dry with a soft cloth.
Try to avoid wearing artificial fibers in linen.
You can apply powder form of lamisil available OTC if there is a problem keeping the area dry with the Lamisil cream.
A swab should also be done ideally before starting the anti fungal cream. The swab will identify the responsible mold and also tell us which anti fungal is best to use.
Hope this helps.