Sudden appearance of itchy red bumps (rash) on stomach can be mainly due to contact dermatitis. This can be due to any new clothes you must be wearing or contact with any irritant substances like skin lotions or creams. This can be either due to direct irritation of skin or allergic reaction to certain substances. If you could find any irritant material which can be the cause promptly remove it and avoid any further contact.
Certain food stuffs can cause allergy and rash, so please check what you had for meal today. Insect bites can release certain chemicals into skin and can cause allergic rash. Check for any medications you have been taking, as drugs often produce allergic reaction which can present as sudden itchy eruptions all over the skin.
Infections like impetigo, scabies and auto immune diseases like Lichen planus can cause itchy rash but they do not suddenly appear over such large area of skin. Observe the rash and notice any extension to other areas as many allergic reactions spread all over the body.
Apply calamine lotion as it soothes the skin and reduces itching. Anti-histamines like cetrizine can be taken orally if itching persists. Steroid ointments can be useful in reducing the inflammation and rash. Visit a Dermatologist if rash is spreading quickly or if itching gets more severe.
Thank you. Hope this helps.