Thanks everyone for your replies!
I guess you're right and it's nothing to worry about, at least I hope so.

Luna isn't doing this very often so I think it's nothing wrong with her.
You have to know, I had a cat that was seriously ill during the last couple of years, I lost her last year and it was very hard for me. Maybe this is the reason why I'm so worried about Luna all the time, it's still in my blood...

Kirsten