I've been very inactive for a period of about 2 years. recently, i've started moving more and exercising. now whenever i exert more energy i get rash?

Thoughts: Although you developed a rash it sounds like it is primarily associated with being active. You do not complain that it itches. I would think this rash is somehow associated with vasodilation from exercise. As long as the rash goes away and you do not have other symptoms I would continue with your activities. I think you will find this rash will finally stop when you get back conditioned.