It is possible that she has Irritable Bowel Syndrome or else is allergic to some of the ingredients in her food. If you are not up for having her tested at the vet for allergies. Then I would try feeding her a food designed for sensitive tummies or a limited ingredient diet like the ones made by Natural Balance. Do not switch her food suddenly though because that will cause diarrhea too. You need to gradually mix in the new food with her old food until you eliminate the old over a couple of weeks. However, it is also possible that she has other intestinal parasites that wormer would not help like coccidia or giardia. I would have a stool sample tested by the vet. Especially if there is an extrememly foul odor associated with the diarrhea. Giardia can clear a room with the stink it causes.
"One cat just leads to another" - Ernest Hemingway
My cat has diarrhea and is eating extremely well and begging me constantly for food. No vomiting. She is also very gaseous and has diarrhea again. The vet thinks she probably has an IBS (eosinophilic gastroenteritis) or Parasite infection (tapeworm). Her white blood cell count is elevated (her Eosinophils), so we are going to de-worm her, give her Vitamin B, a low dose of Prednisone and the new 2-week lasting Penicillin injection (Metronidazole).