Cats- Vomitting

Question
My female desexed 11 year old cat has gone off her food. I have introduced her to IAMS and Whiskas optimum, but she hardly touches it. Previously she was happy to eat cheaper brands. Also she only wants canned tuna for her dinner. She hisses at us if we want to touch her or pick her up.She has started to eat grass and vomit it up.
What do you think?  

Answer
I would recommend a trip to the vet for an exam and blood tests.  At 11 yrs of age, it is likely that there is some type of medical problem brewing (kidney or liver disease for example) and not just her being finicky.   You can try some meat baby food or boiled chicken instead of cat food for a few days to see if that helps slow down the vomiting.