Rash.

I started potty training my 2 year old son a week ago. He hasn't had any accidents during the day or at night in 3 days. All of a sudden I noticed just now that his skin seems all rashy right wear the edges of his underwear are. At first I thought it might be the elastic but there's only elastic at the top and there's no rash there. The underwear are 100% cotton and I have used the same laundry soap since he was born with no problems. He's barely ever had a diaper rash before so I'm just wondering what could be the problem or what I should do!? You would think he'd get a rash from sitting in a wet diaper, not clean dry underwear!