No need to worry. If you had adopted her, the kitten should had able to feed/poo by itself. Just give it sometime to adjust itself. Or try to stimulate it to poo by rubbing tissu paper on it behind gently.
I rescue a kitten 2 week ago and it hardly can walk. Now the kitten can start feeding by it own but still need to feed milk by sucking milk from rolled cotton pad.
Look at how he eat the kibbles. The whole bowl is all his.....
http://www.youtube.com/watch?v=EtQ9sRXp4HQ