Did the Vet check her for constipation?. Usually they can feel if her colon is full. Anyway, yes, there are a couple of things you can try. One, you can try adding a little plain canned pumpkin to her canned food. Maybe 1 teas. per meal. Some cats really like it, others don't. So since she isn't eating all that much anyway, if she doesn't like it, don't try to force it on her. Second, you can use Miralax, or a generic of it. This is what I use whenever one of my cats has issues. I use between 1/8 and 1/4 teaspoon dissolved in about 1/4 teaspoon of water. Then I simply mix that into a little bit of food. Once they eat that, then I give them more food. Miralax has no taste, and doesn't gell up, so it's really easy to get them to eat. I gave my old guy this concoction twice a day for over a year just to keep him regular. If I missed a day, he became constipated, but he had kidney disease, to was prone to dehydration already. It might take a few days to start working though.
I'm also going to refer you to this thread which discusses not only Miralax, but Slippery Elm Bark. I had never heard of using SEB for constipation, but there's a lot of discussion about it on this thread: http://www.thecatsite.com/t/270597/slippery-elm-bark-wet-food-managing-constipation
I'm also going to refer you to this thread which discusses not only Miralax, but Slippery Elm Bark. I had never heard of using SEB for constipation, but there's a lot of discussion about it on this thread: http://www.thecatsite.com/t/270597/slippery-elm-bark-wet-food-managing-constipation