What could cause a small red bump on my stomach?

Red bumps. If you mean tiny red bumps that are permanent, they may be harmless cherry hemangiomas which are blood vessel growths that run in families. They can be removed but there is no reason to do so. If you mean a rash that just came on suddenly, it could be many things such as folliculitis which is an infection or irritation of hair follicles openings.
Rash on Stomach. A rash may show itself as red bumps on the stomach or other parts of the body. Rashes can be red raised bumps, flat red spots, itchy patches, or scaly skin. Stomach rashes can be caused by a number of things including something around the stomach or a body wide condition. Allergic reactions to soaps or lotions can cause red bumps on the stomach. Food allergies, insect bites, and reactions to meds.
Varies. Many conditions could cause a small red bump. To properly determine what it is please consult a dermatologist.