1) Anal/vaginal fistula created from an abcess in the rectum, and the infection pushed a channel to the vagina.
2) A tear in the lining between the rectum and vagina.
It could be caused by anal sex or sex play that may have been too vigorous, or in a tear from the vagina to the rectum through vigorous sex play.
3) A tear in the lining between the rectum and vagina from Birth of a baby, or surgery to your abdomen, or a colonoscopy/sigmoidoscopy
I'm only thinking of possibilities....but if would be very serious whatever the cause. Please get yourself to an emergency ward or your doctor very, very soon. Feces doesn't belong in the vagina, and will easily set you up for infection and all sorts of complications.
I'm sure you'll get other suggestions
It's all a matter of perspective!