Heat rash or allergic reaction?

Two days ago, my daughter had a bit of this rash at the nape of her neck. We haven’t changed anything, but she did wear a new headband that day. I went to change her a little bit ago and she rolled onto her belly and I saw this! 😩😩

It’s spread to the rest of her back, but I’m not seeing it on any other part of her body. Any ideas on what this is? She has no fever and doesn’t seem bothered by it. Thanks so much!