It's something to avoid, definitely. (For example, women having anal sex are cautioned not to let the penis into the vagina after it has been in the rectum for this same reason.) But the vagina is a self-cleaning location. If you allow its natural juices to drain down, and wash your genital area on the outside (don't douche, since you might accidentally push the diarrhoea further up inside you with the douche liquid), you might squeak through this. If your area begins to get sore or irritated, see a doctor.