Have you taken Muffin to the vet for her first check up since you took her home? I know some breeders are good with initial kitten care, but she does need to get her vaccine boosters as well as deworming, preventitive care, etc., etc, at least she is 6 months old and then yearly or bi-annually. Kittens need much attention and medical visits just like a child. Please don't avoid just because vet bills are expensive
. Especially if you got her before she was 16 weeks old. She probably only had her initial vaccine and deworming IF ANY. If not, she should start soon. No matter how careful breeder say, it is very common for kittens to have parasites. And if the breeder said she has been dewormed, deworming medicines are not universal to all parasites commonly found in kittens. And depending on the parasite, we deworm more than once. Also, I hope she was FeLV/FIV tested, too.
Is she having diarrhea or soft stools? Is she vomitting? Lots of things can cause runny stools in kittens. If she is vomitting often, that is really a concern that you should seek medical help. I recommend going to the vet if diarrhea continues because they can get dehydrated very easily.
In the meantime, you could give her Pedialyte, feed her highly digestable food. Kitten food should be fine, just don't change brands too much in short period of time. How is her appetite?
As for age, most kittens do okay being weaned off at 8 weeks old. I really don't think you need to be that concerned about the age now unless she is 4 weeks old... I am sure you are doing everything you can to be a great mom for Muffin
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Let me know if I can help any further.