Aww I'm sorry to hear that Zelda is having tummy problems.
I also think it could be the kitten food as the same thing happened to my Katie when she would eat Mooky's kitty food and vice versa when Mooky would eat Katie's food.
Such a good girl Zelda in taking your butt bath. Kisses for you sweetie.
I hope you are able to come to a solution with your problem.
For me, I had to put Mooky in a room with all his things and then let him out when I got home. Put his food somewhere so Katie couldn't get it, bring Katies food in the same room I was in and when I would see Mooky wanting to eat, bring him to his food.
This went on for months and months until Mooky graduated to adult food. But it was worth it to not have them both land up with tummy problems.
Good luck.