What is this rash?

[EHCP22]

Hi does anyone know what this rash could be? I’m currently trying to contact 111, but the wait time is up to 1 hour. The rash fades under a glass, but I’m still concerned.
DS is 2 and 9 months, has 38 degrees temperature, but other than that in good spirits. Thanks for reading

Heat reaction? Warm toy, hot water bottle, radiator?

I was going to say the exact same thing - that’s what my leg does when I’ve had a hot water bottle on it

That’s reassuring, he did have a hot water bottle before nap time. Thank you very much for your replies.