Here are the splots
And this is the blot
Last time this happened, we assumed she'd burnt her foot on a red hot footplate on a blisteringly (literally!) hot summers' day. That's definitely not what happened this time. It wasn't there in the morning, we put her (very close fitting and never marked before) socks and splints on in the morning, and her foot was fine. Took them off in the afternoon and her foot was still fine; ten minutes later her heel looked like this.