3 Month Old Kitten, Possible Health Concern..?

Two days ago, I adopted a brown and white kitten (two months old) and a black kitten (three months old) from the Animal Welfare League, a pretty well-known animal shelter. My question concerns the black kitten. When animals are adopted out, they have been through their first set of vaccinations and whatnot, and have been deemed as healthy. The black kitten is a month older than the brown/white, but is much smaller in weight. She has no problems with eating, and is a very active, playful kitten. I noticed today that her stool has a rancid smell to it, much worse than the other kittens. After doing some of my own research, it seems that worms are the cause of foul smelling stools in underweight cats. She has had the vaccs for worms, and was cleared to adopt out, so it couldn't possibly be worms. Could there be any other explanation for this?

I think a veterinarian visit is in order. It could be a range of things, even though your little black kitten has been dewormed, there still could be some worms left over. Also, it could be the diet the shelter was feeding her. It is always good to give new kittens a vet checkup just to make sure they're perfectly healthy. Congrats on your new kitties!!!

there isn't a vaccine for worms.  On their sheet they should have been dewormed with "pyrantal" or "panacur"  or "revolution".  I would 

call your vet and ask about the first 2 things.  If they haven't been dewormed, you can always call the place you got them from and

ask for the doses of dewormer.  

Cats' poop smells different, just as is the case in humans.  And a strong odor (and sometimes some color clues) can indicate an intestinal microscopic parasite, such as giardia, coccidia, etc.  A good vet check can help, but it's not always easy to find the infestation with a single fecal exam.  Check with your vet, if you have any doubts.
 

Cat poop odor is also dependent upon the kitty and what you're feeding.  Also, female kitties tend to be overall smaller than males, so I wouldn't worry too much about her size.  However, she definitely needs to visit the vet for a fecal and deworming.