Rash, looks like a flat birthmark, growing very slowly?

A few years ago I developed a quarter-sized rash between folds of skin on my stomach. I didn't think it was anything, except over time it's started to grow away from the fold site, in an asymmetrical way. It doesn't itch and it's completely flush against the skin -- kind of looks like a birthmark actually. I'm only 22 and I never ever tan, so I don't think it's cancer, but has anyone experienced anything like this? Should I be concerned, or is it just due to skin fold irritation?