Here's an update:

I called my vet this morning and explained the situation, but she said what happened was nothing unusual. She said with the dosage she's injected, it's very likely that she's losing a lot of fluid, so the weight loss would be a normal thing.

As she told me before, she wouldn't do that with cats that do not drink much, but Luna's usually drinking well (and she did after the treatment!), so I'm not too worried about dehydration. My vet thinks that Luna will have her weight back in a couple of days. Besides, so far it seemed to have worked, she doesn't seem to have any problems atm.

Okay, now we're waiting for the result of her urinalysis, but we won't have it before next Tuesday.

Kirsten