Have any of you moms seen this before? I thought it was just a heat rash... he got it about a week…

Have any of you moms seen this before? I thought it was just a heat rash... he got it about a week ago, got worse for a couple of days then better. This morning it looked great and wasn't very red but now looks like this! I'm wondering if it's just more irritated because he spit up quite a few times earlier, or if you other Momma's think it's something else? This is my first baby, he's two months old, and I honestly never had any experience really with babies until I had him so this is all very new to me!

Any ideas or tips to help him would be great. He has an appointment Tuesday with his doctor. Not sure if I should go tomorrow if possible or if there's something I can do for tonight/tomorrow to help.

Thanks!