Is she nauseous? One way you can tell is if she wants to eat, she goes to eat but sniffs the food and licks her lips and walks away. I never knew this sign until my one cat became very ill. Now I have picked up on it on several occasions with the cats.
I too think blood work is in order for your cat. You need to rule out illnesses, etc. But the one thing with digestive upset is the tummy hurts so it makes the cat not want to eat. Sounds to me like she has IBD (one of my cats has this and also the new kitty is exhibiting symptoms) and your kitty may need something for inflammation. I am consulting with a really good holistic/homeopathic vet. We are using homeopathic remedies, NAET and JMT. It sounds like your kitty may have severe allergies and until you find out what allergies she has she will continue to have reactions to the food you feed her. NAET is a form of allergy elimination that I do at home. We have found out what my cat is allergic to and I treat him at home. If you want to know more about it please ask
I took in a cat in mid-January that ate her food fine at first but started going off it. So, I used some grain, soy and gluten free fancy feast as bribe food (it was the only food she would actually eat) and now we are dealing with soft stool with blood. Before she came to me she was eating mostly dry crappy grocery store food with a tsp of wet food a day. But when she broke her tooth on the dry food I took her off of it (which was my plan anyways) and feed her strictly wet. I went back to feeding her the crappy wet food. She too is VERY picky and she still has loose stool with blood. I have tried everything to get her onto a higher quality wet food but like I said she is really picky. The crappy brand of wet food has the same flavour but different forms (pate and chunked). I have to switch between the two types often because she too will stop eating the one form all the time. I am finally getting her to eat some better quality food....I have been adding a sliver of the new wet food at the bottom of the crappy wet food and sprinkling crushed up dry food (I use a pill crusher) on top. She's been eating this nicely lately. I am very slowly increasing the new and decreasing the old.
Other things I have to do is warm her food up or else she won't eat it. When I feed her the pate style I have to make it a really mushy consistancy (I also like to add a bit of water for hydration) or else she won't eat it. I also have to feed it on a plate and not a bowl or else she won't eat it. For all my cats I feed them wherever they will eat (does not have to be in the kitchen only). Whatever spot they want to eat I will put the food down there for them (may even be on the couch)....I gave up on the kitchen only thing when I was switching my cats over to a raw food diet. They wouldn't eat in the kitchen but if I put it where they were they would chow down....
Some other foods you may want to try that are fairly simple are:
By Nature canned
http://www.bynaturepetfoods.com/retailer_locator/
EVO 95% canned meat
http://www.evopet.com/where-to-buy/
Natural Planet Organcis canned
http://www.nutrisourcedogfood.com//n...s/where_to_buy
Wellness "Grain Free" canned
http://www.wellnesspetfood.com/store-locator.aspx
Some other bribes to add to her food is parmesan, sprinkle some dehydrated or freeze dried 100% meat/fish on her food (like Halo Liv-A-Littles or Whole LIfe pet), boil some meat only and use the broth in her food, Bonito flakes, there's also catnip, Rosie's various "dusts"
http://www.rosiesrewards.com/products.php#rosedust