It's probably the strange smells from the vet surgery which Amber doesn't like. Maybe give Mogs an unwashed jumper of yours, and rub her gently in it to transfer the smells of home. An especially grubby, recently worn one will help. Maybe also rub her in anything she usually uses, like the blanket she sleeps on.
Then try letting her out. If you have a bad reaction from Amber, shut Amber in the bedroom for a short time. This way, Mogs will be able to gather the scent of your household onto her body. Later, try letting Amber out again, and see how she reacts.
It's very common for cats to react negatively to another cat who's just come from the vet, especially after surgery. She's covered in the smell of the vet, nurses, aneasthetic, and all the other strange things, like other cats and dogs. It'll take a short while for Amber to recognise her again.
