What is this on my daughters arm?

Lenny • 👩🏻👨🏼‍🦰👧🏻👦🏼🤰🏻

I sent a picture to my sister (a NP). It’s late so I can’t call her doctor. My kid spent half the day with me, half with her dad. She came home and showed me her arm and when I tried to touch it, she freaked out. It was red and hard to the touch in the red area. I asked her what happened and she has no idea. It looks like a burn to me, but she doesn’t remember anything happening to it, She didn’t complain about it at all today with me or with her dad.

The first pic was taken at 9.

The second and third at 10:30. So, only an hour and a half difference.

Any ideas? It looks like a burn to me, but it’s just odd that she doesn’t know how it happened .