Rach welcome to our world
It could be an allergic reaction to something in the diapers. Don’t know if she’s getting up and out of the house with them. If so, maybe try different brands. Also you could try putting a strip of material between her and the diaper. Say try it at the waist, where it should stay dry and see if that helps. If so, you’ll know it’s something in the diaper. Try other brands etc.
If she’s not leaving the house, you could do as my husband does, he calls it pants optional. She could be commando under her blankets. You can get cloth bed pads on Amazon. We’ve used them for years. Easy to wash. If she still has control of her bowels and can tell you when she needs to do, then add the bed pan and a green paper pad under neath for a short bit. That will help eliminate the worst of it without it being on the cloth pads where the paper/plastic would only be against her skin for a short bit.
If you determine it’s definitely a reaction, and she needs to leave the home with clothing on, then I suggest looking for adult cloth diapers. I’m sure they’re out there, but haven’t looked myself.
I really like coconut oil for the skin. Works well for DH. However there ,igniting be a bit of fungal mess too so you could try monostat cream or athletes foot cream on the rash as well.
Hope this helps
Sue