What is this?

FarmWife 🌾 • 2015 👧🏼 2017 👼🏼👼🏼 2018 👧🏼 🌈 2020 👶🏼
I noticed this after I went swimming over a month ago now. It doesn't appear dry. It comes and goes with how bright red it is. It's on the inside fold of my elbow crease.