New Prescription Diet woes

Andy had a bout of vomiting/diarrhea yesterday and last night. Took him to my former vet (farther away but worth the drive) and he was given an injection for the vomiting/diarrhea and Centrine and Metronidazole pills to take for this week. He also is to go back on Lowfat ID, which he has been eating this year with good results. AND, most importantly, he will be tested for Cushing's disease during the next few week. I've read about this and would appreciate any input or information anyone could supply. This is NOT what I expected - I thought I was just dealing with a mast cell tumor and elevated liver enzymes.

Rita