Diaper rash?

My almost 12 week old had a really bad diaper rash around 4 weeks and the doctor gave us nystatin and it cleared up and she told us to keep diaper cream on as a preventable measure after we stopped using nystatin. Wellllll since then, we keep diaper cream on him at all times. Is this the wrong thing to do? I looked at his bum today and it’s red and has little white bumps but it’s only in the small area in his cheeks, not past that. I plan on getting him more nystatin to clear it up but could it be because we constantly keep cream on him?