Newborn diarrhea and rash

FTM- I have a 5 week old boy. I was doing combo breastfeeding and formula... until this past week I decided that I need to quit the breastfeeding due to it affecting my mental health. I have only breastfed him a handful of time in the past 48 hours. A rash started on Monday 4/28 and it has gotten worse to the point of a blister and still hasn't gone away because he is pooping so often. And with pooping so often, it's considered diarrhea and it is making it hard to heal the rash. Google says with newborns with diarrhea, there's the risk for dehydration and the way to prevent dehydration is more feedings... BUT... more feedings will just make him poop MORE... so is it just me or does that not make any sense? And what can I do to figure out what is causing the diarrhea because it can be ANYTHING... how do we narrow it down fast enough to stop it from causing constant diarrhea.