Ditto what Opus says. I've had cats who jumped off the refrigerator after being spayed with no ill effects. You just want to be sure she hasn't gone somewhere and chewed the stitches off. (I had one girl who needed to be resutured with steel wire stitches because she chewed hers off twice!) The growling is doubtless because she feels sore and disoriented and wants to protect herself. She should come around in a few days.
no a collar certainly isn't necessary unless she is licking the area alot an its becoming inflamed.
I would think her change in attitude is due to trauma and some pain and should become herself in a day or two, if not I would contact the Vet.
Is she still eating and drinking as usual?
She really shouldn't be jumping up and running around for a few days....however I've had many who do and don't cause any damage to the incision...just try and get close enough to her to try and have a look by tomorrow(hopefully she'll be more settled by than)
Please let us know how she makes out ok, or post again even if you have more questions.
♥Opus