We use Slippery Elm Bark. You can get it at the health food store or Loblaws. It's a capsule but you can break it open and put in the cat's food. It's sweet.
We gave it to our last dog who had Colitis and I swear by it. They also use it in people who have IBS and Colitis. The vet was amazed at the difference. We gave it to him for a few weeks and then only when he had flareups. It's completely natural and no side effects. You can look up different sites for it on the internet. It was a breeder who originally told me about it and I would be lost without it. Appparently it soothes the insides and stops inflammation.
I was giving the dog's ( 80 lbs) three in the morning and three at night.
I would still take your vet to the vet first though to make sure that's what she has just so it won't be covering up anything.