Hi Sierra,
It sounds like she is having trouble getting the milk from the nipple. Try and open up the nipple by putting a X in the rubber tip. When the kitten sucks, it opens up more than the pin hole they tell you to put in there. Once she starts getting more milk, she will relax and settle with the nursing I think. You may also want to feed her a little more often, and try putting a FEW DROP of olive oil or mineral oil in her milk to keep things soft when she is ready to poop. She may have been malnourished, and not have a lot of poop in there. I also recommend switching from KMR to goat’s milk as it just won’t cause constipation like KMR does. You can get it in the grocery in the milk section as fresh, or in the baking aisle in cans and powder. The canned is condensed, so add water 1 to 1. It works miracles in my experience.
I am going to post some links for you on hand rearing kittens, I have used them many times over the years. They answer lots of questions that kitten mommies have. And a link for the ‘triangle’ method of pooping a kitten. Let us know how you both are doing!
http://www.safehavenforcats.com/newborn.htm
http://messybeast.com/handrear.htm
Helping a kitten to eliminate