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Hello. I'm sorry about the long wait.
It sounds like she may have picked up an URI (upper respiratory infection) when she was at the vet. This is not too uncommon at all. It happens with kids at the doctor office as well.
URI's can often times cause mild laryngitis. This may just cause a change in meow, lethargy and gagging. The gagging can look like retching. Most of these will resolve on their own over 1-2 weeks. As long as she is still eating/ drinking, then I'd let it resolve on it's own.
I hope this helps, let me know if you have any other questions.