Red circle markin a glass

So, my bestie, my sibling and I have always had weird stuff happen around us and just today happened something odd and I just thought of posting somewhere where people might have an explanation.

My friend got a clean cup to drink water from and after drinking a red ring appeared at the center of the glass, like where the water met the glass. She wasn't wearing any lipstick or anything, it's water we've been drinking from a bottle so it has nothing weird in it.

I'm curious if anyone would have an idea of what it might be.