First off I would not have switched his food being he just went throught a tough surgery as that is a tough surgery
You should give the body time to heal completely until doing different things.
What meds was your dog on while recovering was it metacam, rimadyl, deramaxx or was it tramadol for pain relief? Nsaids can have side effects which are metacam , rimadyl nd deramaxx
It could also be the food what is the fat content on that food as yorkies need to be on a lower fat diet as they are prone to pancreatitis but if it was pancreatitis your dog would be restless, lethargic, and in pain.
This breed is very sensitive stomach wise so when you throw too many variables into the equation you will have a hard time narrowing down with issue so in future try not to do a bunch of changes with them as if you would not have switched the food we would know for sure it was something with meds and surgery but now we have food switch in equation so have to rule that out as well. Most likely the food is too rich as that was the recent change
let me know more and you need to compare fat content on old food vs this food as that will help