As doodlebug said, you need to make sure she is getting fluids. If she has diarrhea, it could be due to one of several reasons, including food digestion problems, infection, or worms.
Her rectum could be protruding because it is swollen from the irritation of continued diarrhea. It could also be protruding because she is straining so much to go poo. Sometimes constant straining and producing small amounts of runny poop with mucous are actually signs of constipation or an impaction that can't be forced out. The rectum can actually prolapse, and that isn't good.
Either way, she needs to be kept hydrated and she should see the vet just as soon as possible. Hope all goes well with her!
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