Has the kitten been stimulated to defecate ok since feeding the formula? Be sure if you are feeding more by the bottle that you do this and all is coming out ok. Sometimes if you over feed and then nothing passes it can cause problems. Sounds like the little one is really happy to feed though that's a good sign! Also nice to hear mum is letting it feed too. Sometimes it helps to keep her with you if you feel she's going to bring the kitten away from the nest. Maybe put her and her kittens in a kittening pen or dog cage for a couple of days. Letting her out every so often to get some space, use the loo, have some food and excercise.