Kits need to be kept warm or they can’t digest food. Make a nest for her using soft blankies and something with your smell on it. Place a heating pad, on LOW, covered by a towel then put your wee kit in her nest. Be sure to provide a “cool” spot so she can to crawl to if she get to warm. Since she is an “only” child, place a small squishy toy in her nest with her. She will snuggle up to it.
When feeding, do not hold the kitten as you would a newborn baby. They must be placed with paws on the ground and fed exactly in that position. This reduces the chances of getting milk in her lungs. The formula is given at body temp, never cold, through a nursing bottle.
The third thing to realize is Kits do NOT come with the knowledge of “going regularly”. They would, under ideal circumstances, have their moms clean their bottoms and help them urinate and defecate. Well, guess what? You’re mom and I’ll provide a link on how to do it.
http://messybeast.com/catarchive.htm
https://www.youtube.com/watch?v=enJrSU-nqCc