I had all 4 of mine done when they were around 5 to 6 months old and I worried like crazy too, even though I knew it would be a fast operation and they'd be all right. When I went to pick up some special flea stuff this weekend, the girl at the front desk of my vets office told me the operation itself doesn't take more than about 15 minutes, so they really aren't under for a really long period of time.
You love your girl and it's understandable you are going to worry. The blood work is a great idea. I had that done to 3 of mine and will have it done to the last 2 soon, when they are a year old. I think it's a good way of being sure they're healthy.
Please let us know how she's doing later.
There are only two rules at my house: House rule #1. Cats rule. House rule #2. See rule #1.