12 weeks old kitten

I just adopted a two kittens a brother and sister and i have a vet appointment scheduled for next Sunday for both of them.  When i got them, they were fine, eating normal and drinking normal now the brother is starting to be really picky with food, today he only ate twice but i noticed he started drinking a little bit more then usual.  Then he used the bathroom and he vomited.  But he still plays normal with his sister. This is my first time owning a kitten so i don't know what to do. I'm getting concerned

Hello - I'm afraid think that you're right to be concerned.  Kittens, on account of their size, dehydrate extremely quickly and it sounds as though yours needs to be assessed in case he need fluids.  There may be some innocuous explantions for what you are seeing, but your vet may also be concerned about eg kidney or liver disease, so an early appointment may be preferable.   Perhaps you could call your vet or their emergency counterpart as its New Years' Day, and ask for telephone emergency triage.

Typo with apologies - I'm afraid I think that you're right to be concerned. Wishing you both all the very best.

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