I have recently developed skin rush at the back of my neck, arms and stamac. what might be the cause?

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.