An update (as posted in Cat Health also):

Thanks everyone!

I've found out something very interesting today! After Luna got that anti-inflammatory shot yesterday, her stool was NORMAL this morning, plus it's been the first time for many days that she was using her box again!

I haven't changed her diet yet, I first wanted to find out if there would be any improvement by the shot only, 'cause this should give the vet a hint. She's not in her office today, otherwise I would have called her already to tell her, but to me it looks very obvious now that there must have been inflammated areas in Luna's colon already. Clostridia is well-known for causing colitis if untreated and it looks like Luna's colon tissue is already damaged.

I will feed the last can of her old diet today to see what happens, than change her diet as discussed with the vet. On Monday, I will call her again to discuss further treatment. I hope this means we're on the right path now!

Thanks everyone for your support and advice, and for keeping my girl in your thoughts!

Kirsten