Two days ago I noticed a light line on my child's belly, and today, after a closer look, it turned o…

Two days ago I noticed a light line on my child's belly, and today, after a closer look, it turned out not to be a line — the two areas on the belly are reddened, like a small rash, and when touched they feel like tiny blisters. We haven't given anything new to eat, and we haven't changed the laundry detergent. What could this be, anyway? I'll take another look tomorrow morning and schedule an appointment with a dermatologist. Photos are in the comments.