I'm no expert on this by any means but I have done quite a bit of reading on vaccinations and boosters simply because my Isaiah (boxer boy) had a really bad reaction to the lepto when he was a pup, my vet never gave it to him again after that. Now that we have Maggy May and she'll have to be getting vaccinated when the vet and I both feel her immune system is back up to par (she was really sick cause she came from a real bad situation when we got her) anyhow, I'm scared to death to get her vaccinations. I've expressed my concerns with my vet and told him how scared I was and he said he agreed with me in waiting for her to get stronger and her immune system to built itself up and then we'd vaccinate her. He knows everything she's been through in her short life so he had me to scheduel her appointment for first thing in the AM on a Monday, that way if she had any kind of reaction, I could rush her back into him. In doing research though, I came across this:
http://www.leerburg.com/vaccinosis.htm
it talks about pets being over vaccinated and having reactions to it. I'm in now way saying this is what's wrong with your lil one, but it still doesn't hurt to read it and perhaps talk to your vet about it. It could simply just be the change in her food, hopefully it's nothing serious. Please keep us posted about how she's doing and what the vet says.