Sick Foster Kitten - PLEASE HELP!!

I have a foster kitten who has recently become very sick. She is 6-8 weeks old and is the only one showing these symptoms out of a litter of 4. About 6 days ago she began vomiting her dry food immediately after she ate. She also seems to get very mucus-y and congested after eating any dry food (currently eating Royal K9 - baby-Cat). She seems to dry heave and have a difficult time breathing. This lasts for a few minutes, and she eventually vomits up the few kibbles she managed to eat. It looks like she still has an appetite, and is drinking water and using the litter box, although she is rapidly losing weight. Her energy level is very low, mainly rests, and doesn't interact with her siblings. So far we have tried a high calorie supplement and kitten formula, which she doesn't seem to like. Does anyone know what this could be? Or more importantly how to get her better? If I take her back to the humane society, I'm afraid they'll just put her down. Any help would be appreciated greatly!

[opus88]

I'm so sorry for kitties problems, and bless you for trying please don't take her back as you say they will put her down...give her a chance for good care please.

I think she needs to see a vet, it may not be related to the food at all???
vomiting can be a sign of so many other problems especially when she is also mucusy and congested when did she have a check up??

Can also be an allergy to this food that has developed?
Or is the kibble to large for her and she is choking?
Why do you not try another food???
Or at least a wet food? that maybe easier for her to tolerate.
I think that would be a place to start and if she vomits after a wet food too than she surely Needs to see a vet.

please keep us informed...and please keep trying
and the best of luck to you both