These rashes can develop from a lot of different things. My LO had kinda what you describe for several days due to the rest of the pregnancy related hormones leaving her body (so I was told). Looked a bit like a heat rash.
Most other rashes are just fine. The one to watch out for looks like this and is accompanied with fever:
https://news.bbc.co.uk/olmedia/1880000/images/_1881288_menin300.jpg
If it doesn't look like this and baby doesn't develop a fever, get lethargic, etc. I just used aquafor lotion on her to keep any possible itching down and kept an eye on it.