It could be a number of things but obviously doesn't sound good. She is probably gasping for breath from weakness or a viral infection.
You can try Chick Saver, but as she is not growing and thriving, it is likely something else going on.
It could be coccidiosis, but usually they sit huddled with runny diarrhea (not always bloody).
Sometimes the failure to thrive is from residual bacteria left from hatching that has invaded the abdomen. If she has had a continuous low grade infection, that could account for lack of growth, loss of appetite, which accounts for less poo.
If you put her on Sulmet, that acts pretty fast and would address both coccidiosis and a number of the gram negative bacteria that are common from hatching issues (eColi and Salmonella).
You could also try a broad based antibiotic like Tylan or Duramycin-10, but if it were my chick, I'd put it on Sulmet to see if that helps rally it. You can add it with the Chick Saver. If that doesn't work, then try the Duramycin or Tylan in case it is secondary infection after a viral (any weepy eyes?)
Good luck to the little one, but I must admit it is rare I am able to rescue one of these, and often you may not want to as they grow into weaker birds that fail to do well and simply succumb to something else as adults (which can pass to your flock).
Edited by Lady of McCamley - 4/28/16 at 1:07pm