Random unexpected flare up of redness on back us bottom

It's along the crease of her butt. If I were to lift her legs up as if I was changing her, you can clearly see it.

Imy not sure if it's bothering her or not but she seems okay. She is on medicine so I thought maybe yeast? But it looks like a diaper rash as well. Anyone experience this?

I'm using AD OINTMENT