What is this rash?

Olivia • SAHM 🤰🏽 10/17/2016 👦🏼 🤰🏽 8/31/2018 👶🏼 Carter and Leo ❤️

He’s had the chicken pox vaccine? He kinda seem to itch it... he did wear a new shirt that I didn’t wash first but I think it was already starting before he even wore it.