My baby is 4 months old and just got a partial dose of her first set of vaccines

She’s been feeling bad since she got them, just fussy and a slight fever nothing to worry about. Last night when I was changing her diaper I notice she had to red spots where this rash (?) is. It didn’t look like this last night it just looked red like how your leg looks after you’ve rested your arm on it for too long. So I put her rash cream on her.

We wake up this morning and her little bottom looks like this

I will of course be calling her doctor first thing Monday morning but there’s nothing I can do until then so I wanted to ask if anyone else has had experience with this

* We have NOT changed diaper, wipes, soaps or formula brands since she’s been born*