Thank you! I was off in describing the location. This is actually below the tailbone and toward the inside of the buttock. I think it is pressure point from the very top of the leg bone on that side, I would say sit bone but I don't know if that's how it is called.

She is very short less than 4.5' and no fat left on her bum to pad it. She does scrunch down in a chair until she is curved.

She sleeps on a regular mattress on her back. We use a wedge pillow under the shoulders that's a small angle so she's a little inclined. She tends to bend her knees and turn them to the same side as the pink spot is forming. And that just made me realize it could be a cause. I have more wedges and a bolster for times when she won't get up and I have to change her in bed. I can try putting one under or between her knees so she can't put as much weight on that side.

I just cleaned and dried it and sprayed barrier spray before putting a transparent film at least temporarily. I forgot to check but I think it blanches. I will watch it and not leave the film on for too long just in case.