Thanks Pieper's Mom! Luna is doing much better now! I went to the vet on Monday and she said that it could be possible that she played a bit too wild and her belly was aching, but 10 days after a surgery nothing bad could happen. And Luna already felt much better the next day!
She is still very young (5 months), too young IMO for that surgery but she already started peeing into my bed, obviously a first sign of the heat. When I spoke to the vet after the surgery, I was told that there have already been a beginning ovulation. I think she was really young, wasn't she? But now she's doing fine!

Kirsten