What could this be?

My LO had this random rash pop up 2 days ago. Didn’t seem to bother him and didn’t get worse as the day went on. I gave him Benadryl just in case at bedtime that night. The next day it seemed to be a little better so I didn’t give another dose. Tonight after bath time I noticed it looked like it had spread a little. He has had no fever but has been super snotty. We haven’t changed laundry detergent but did try oatmeal mixed with formula twice but he had less than a tablespoon (babe isn’t ready🤣). I thought maybe it was from the diapers we sent to school bc they are a new brand but it’s under is arm too 🤷🏻‍♀️. Here’s a pic from tonight after bath of under his arm and thigh.

I plan on calling his dr if it isn’t better tomorrow.