I know how you feel. I was a nervous wreck when Molly went to get spayed, too. We had to sign a contract at the pound to have her spayed at the humane society, who has a horrible subsidized spay and neuter program ($20).

My worrying was called for it turns out. 4 months after her spaying, she had an infection from dirty sutures, and she has just gotten over a false pregnancy. Turns out she wasn't completely spayed.