It does sound like it might be something in the food that disagrees with her.
Really you need to do an exclusion diet over 8 weeks under a vet's supervision. No treats or anything else at all to really rule out an allergy.
James Wellbeloved is very good but some Cavaliers don't like it!! Burns and Fish4dogs are also good.
Mine are now on some raw and some Markus Muehle which was the best kibble I could find.