Rash?

Has anybody seen this type of rash before? Our 2.5 month old just started this a day ago. The rash is constantly changing, as in it will go away and come back in a short amount of time. It’s slightly raised with tiny bumps and slowly starts to disappear once it reaches his chest. I noticed it’s worse when he wakes up, especially on the side that was laying against the mattress. Could this be a reaction to our detergent or maybe baby eczema? Other than this he seems absolutely fine.