rashes.

My 7 month old son has a red area from the top of his diaper and the creases of his thighs to a little on his bottom. (Including in between the areas I mentioned) It's not splotches or a raised rash and no bumps. I put desitin on it about 7 hours ago than again about 4 and it's still apple red he's had it since yesterday night and I'm not sure what to do. Any advice?