Sounds like a case of greedy vet to me. Have you had a chance to get your kitty to another vet? How old was Jasmine when you had her spayed? In the picture she still looks like a kitten. Part of the weight gain could be that she's simply still growing.
I had my three adult cats spayed in November and two of them gained quite a bit of weight. I'd heard of cats gaining after they're spayed, so I just assumed it was a combination of hormonal changes and the time of year. They were also putting on their winter fat. They're indoor cats and we don't get much in the way of winter weather, but they still store up fat for it.
They're starting to drop a little of the winter fat now, but I don't think they're ever going to go back to being skinny little girls.