It could be from stress, definitely. The mucus in the stool is evidence of colitis (inflammation of the colon) which could be from a lot of different things, and stress is one of them. She might not have any of the typical parasites that are tested for (roundworm etc, because Revolution treats for those things too), but she might also have them at low levels. If it were me I'd have her stool checked again for routine parasites, and also for Giardia (ask for the antigen test) as well as Tritrichomonas foetus (PCR/DNA test, not the culture). Get repeated deworming even if the fecal is negative, use a special GI diet for a while, and maybe some symptomatic treatment for the diarrhea other than kaopectate, if it's not working (ie ask the veterinarian about metronidazole). If it's stress-related then work on that too, try pheromones in the house (see Feliway website).
Some breeds are more predisposed to problems like inflammatory bowel disease, too, so you might need to get more diagnostics for the cat. Don't give up, it's not normal.
I have done a lot of book-learnin' on these things.