Hey somy … Its great you have the little one on kitten formula.. What brand is it? I know you say he was getting attached so you had to take him but usually its not recommended to take a kitten from its mother till a minimum of 8 weeks.. Two weeks is extremely early especially considering it wasn’t an emergency rescue situation im only saying this so you don’t have to experience something similar in the future its always best to keep mother and babies together while they are still feeding .. But you did great to get her to this stage 🙂
As for the diarrhea…. At 4 weeks she’s should be old enough to be wormed weigh her and ask your vet about the correct worming meds.. Get these from your vet not a pet store it won’t cost any more and you need to be extra careful when worming young kittens…
I would then maybe try a different brand of formula and at 4 weeks you could even introduce a small amount of wet kitten food to bulk up her stool.
.