It might not be period blood. Endo lesions and nodules can form inside the rectum, sometimes when they are irritated (read: rubbed up on by exiting poo or a penetrating object) they can bleed. I would mention it your dr. it might not be serious (my dr. isn't worried about mine) but it is good for them to know.

And no worries. Bodies are gross. If you can't talk about bloody poo here where can you?