Dear friend,
Here is my concern, your girl may have developed an oral infection due to the cleaning, the stress of the surgery or being boarded.Look in her mouth and see if there are any abnormalities. Even if you see nothing, the rancid smell is one of bacteria. See your vet ASAP for a round of antibiotics to clear this up. For tonight, give her a teaspoon of plain yogurt to get "good" bacteria into her system.
Please let me know how she is and if you have any further questions tonight. I will help in any way possible.
Very sincerely,
Terri