I am not sure who has the more stress when visiting the vet - me or the cat.
We have found a vet practice that is called Just Cats - name self explanatory.
The oldest cat, Paddie, tolerates everything at the vet office, until she has to go back into the carrier. Then she becomes snarling black panther and turns and swipes at whatever hand is nearby. The last time we took Paddie, having explained her reaction to going back into her carrier, they simply unscrewed the screws holding the top and bottom of the carrier together and Paddie stayed on her cushion and they gave her her jabs, and then put the lid back on. Less stress for all concerned.
One of our gang, Ellie (also known as Ellie the Belly) had her jabs recently and the vet commented that she had lost weight. I told him that she is more active since coming to the UK and that her coat is so much better than it was. He thought that her coat was not as good as it could be (he hadn't seen it before!) So Ellie was wormed and we had instructions to bring her back for a weigh in - which we did last week. She is fine (and has put weight on again mainly because we were feeding her up).
I hope your fur baby is fine. Sometimes the simplest of things can make a big difference - as you said, diet. With Ellie the first step was to worm her, then if she hadn't put weight on they would have done a blood test.
Sorry to ramble, hope everything is OK.
