The bathtub bit me?

I am on my period and cramping so I decided it was time for a hot bath to soothe the cramps. I had just cleaned the tub and started to fill it with water and I laid down in the tub and my back like suctioned to it, it hurt like a bitch and my immediate reaction was to get up. IT WAS SO PAINFUL. I yelled and I got out after that and looked in the mirror and there's this GIANT hicky looking mark on my back. It looks like a demon bite, what the hell. This has never happened to me lol