I have a rash which is spreading out. it started on my stomach and has now spread to thighs and neck. it's been 3 days now?

Hard to say: If the rash is moving from a local to a general distribution it is unlikely a simple contact dermatitis like poison ivy. Generalized rashes can be secondary to an infection (virus or bacteria) a food reaction or bug bites. Without examining the rash it is hard to speculate on a cause or specific treatment. If it itches, Benadryl can be given for comfort.

See a doc: Many causes of rash, from allergies to infection, so cannot determine based on this symptom alone. Until the visit, try topical phenolated calamine lotion, and oral anti-histamines to control symptoms. Good luck