Probably the safest med you can use over the counter is one containig kaolin and pectin - this is a binding agent and not a antibiotic. However any medication should really only be administered under the supervision of a vet so it would b worth callling your vet to check that this is suitable for the individual animal.
There is a product available called kapectolin or kao-spen. There used to be one called kaopectatae which is now NOT safe for use in cats du to and ingredient change.
These produts can be given at a dose of one to two teaspoons per 10lb of cat every frou to six hours, orally. You could also combine this with a bland food diet of plain chicken and bolied rice to try to ease the diarrhoea. If there is no resolution within a few days or your cat shows any other signs of ill health, Take her to the vet.
I hope this helps,