My cat has severe diarrhea, any ideas how to cure this?! (Vet didn't help at all..)?

I took my 8 year old cat to the vet a few weeks ago and they gave her antibiotics and some spaz tabs that temporarily helped her but as soon as she quit taking them she got diarrhea again. When I promptly took her back they just gave me more of the spaz tabs. They told me it was stress, she hasn't had any major changes and has been on the same strict diet her whole life. I have 3 other cats and changing her food isn't the best idea, it seems as though I am just making all of them sick that way and it doesn't help her either. I don't want her to get dehydrated in this heat and I am sure her stomach must hurt, is there anything safe to give cats for this kind of problem? Pepto? Immodium? I can't afford to keep taking her to the vet and getting a temporary solution so I really don't know what to do. Getting rid of her isn't an option, I don't feel that anyone else could take any better care of her and don't want to give her away just because of this problem. I keep dry food down at all times and usually give her wet food once a day but have stopped the wet food because of her stomach, is there anything I could give her to replace her wet food and try to get her stomach settled? ***I intend to get a new vet as soon as possible but have already spent hundreds that I don't have on this, I am 7 months pregnant and just want to keep my cat healthy- Please HELP!!!