I'm glad you took her to the vet again. Does the vet think it could be pyometra? Pyometra is a potentially fatal infection of the uterus. If so, she will need spaying after she has her kittens to remove her uterus. Please ask the vet about this. Is she still on antibiotics for the infection?
Also, has the vet given your cat an x-ray to see how far along her kittens are? It might be a good idea to check.