maybe a milk or heat rash?

My first had something similar one time when she was smaller, but I've found out, it was from my fabric softener. I'd keep an eye on it, to see if it bothers her or if it lightens up. But doctor would be my best bet, for sure

No it's just in one spot @bri28 and it is hot here but she has been inside all day in air conditioning @mommyagain

I would call pediatrician and get a next day appointment or if they are still open come in for emergency check up.

Look like heat rash was baby hot or is it hot where you live?

Is it on her whole body? @annr123