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It sounds like she may have picked up an upper respiratory infection while at the vet's office. This is not an indicator of how clean the place is. Kennel cough and URI's are communicable diseases so she probably got it from being in the office with an infected pet or in the kennel. There is not much that you can do to prevent it. The spay probably weakened her immune system and made her body less able to fight it off.
Since the discharge is green/yellow, it means that you are dealing with an infection. You will need to contact your vet (by phone or in person) and let him know what's going on. He might want to give antibiotics.
Let me know if you have more questions.