We had this too, rash got larger and stayed big for about 36 hours then went away ☺️
I'd call for advice as I'm sure we were told it shouldn't get bigger than a 50p
She had a temperature yesterday but not today, but did need Calpol which cheered her up! I found on the nhs website it said if the rash gets any bigger than 5cm we must call 111, and hers is exactly 5cm big! So keeping an eye on it throughout the night to make sure it doesn’t get any bigger x
My daughter was exactly the same x
I'm pretty sure my son's was like that but can't remember exactly how long for. Does LO have a fever? If you are concerned but not too worried then I would call the docs in the AM for reassurance. If you are worried about it and other symptoms then probably call 111 to get earlier reassurance.