2 week old kitten, trouble deficating 1&2, and gurgling

I have 4 rescued kittens. I found then with their dead mama. I got them when their eyes were closed but theyre open now. 1 kitten has had problems since I first got her. Her first bottle feed (powdered kmr) she puked up white foam and formula. I assumed I over fed her, but she hadnproblems cating her breath and I thought she mightve inhaled some but it went away. It happened again same thing and hasent happened since, but when I try to hold her on her back to stimulate her to go potty she starts kinda coghing and gurgling like shes drowning and cant breathe!! Its freaking me out!! The other kittens pee alright, and it comes out in streams but this one it doesnt seem like shes going potty at all. Or it just bearly drips out. She hasnt pooped since I got her a week ago, atleast when im stimulating her. Theyve been somewhat pooping on their own a little in their cage cause they keep pooping on eachother. So I dont know if she has or not, but her tummy isnt bloated or distended, and she doesnt constantly cry unless all of them are. I massage her anus but shes not going, ive heard mineral oil?? Not sure where to get it tho… im taking her to the vet tomorrow but would like to try myself aswell. All the other kittens werent pooping regularlt either but I watered down the formula a bit and theyre all doing better but not her. Ive heard of kittens not actually being able to poop and having sac inside their rectum?? Would I be able to tell that or can only the vet?? I already love these little babies and wanna do everything I can?? Is there a better way to stimulate them to go poop?? Thanks