Thanks for your replies! Well, I spoke to somebody else today with a cat that suffers a kidney disease and she described the same symptoms as you do, 4 feline house. So I think it must be something else that's wrong with my Luna.

Spencer, I will bring some stool to the vet tomorrow to have it tested, her stool's color is brown most of the time, sometimes a bit yellowish. It became better today, she's more playful and doesn't seem to feel so bad but she gained a lot of weight during the past days, even though she is sick.

Kirsten