Very light pink rash on stomach. Does not itch. Turns red when scratched.

Allergy. In all likelihood this is a reaction to something you have been exposed to. It could be a dermatitis from a new cleaning agent (laundry) or other agents you have been exposed to. Monitor rash and if it remains or gets worse consider visit to urgent care. Check and see if soaps or detergents have been changed in house.
Eval. This can't be diagnosed with the information that is available. A history, physical examination & other studies may be needed to determine cause/causes. Once accurately assessed a treatment plan can be developed.