I'm sorry to hear she is declining
At this point the Nystatin won't hurt to try. I will link you to a very helpful article by @TwoCrows about crop issues. She addresses causes and treatment options thoroughly.
@azygous has stated, sour/impacted crop usually is a product of another underlying condition. Swollen/sour crop can also be caused by some type of blockage, whether it's from food, bedding, cocci, worms or a developmental disorder (failure to thrive) that is just now showing up at her age.
Make sure she has some grit available, even though you only feed chick starter at this point, chicks can ingest all sorts of things that may slow the crop. Treating for Coccidiosis wouldn't hurt either. Treatment is with Corid.
Do the best that you can do and hope for recovery, but prepare yourself for the worst.
https://www.backyardchickens.com/a/impacted-slow-and-sour-crops-prevention-and-treatments
At this point the Nystatin won't hurt to try. I will link you to a very helpful article by @TwoCrows about crop issues. She addresses causes and treatment options thoroughly.
@azygous has stated, sour/impacted crop usually is a product of another underlying condition. Swollen/sour crop can also be caused by some type of blockage, whether it's from food, bedding, cocci, worms or a developmental disorder (failure to thrive) that is just now showing up at her age.
Make sure she has some grit available, even though you only feed chick starter at this point, chicks can ingest all sorts of things that may slow the crop. Treating for Coccidiosis wouldn't hurt either. Treatment is with Corid.
Do the best that you can do and hope for recovery, but prepare yourself for the worst.
https://www.backyardchickens.com/a/impacted-slow-and-sour-crops-prevention-and-treatments
very helpful!! She has perked up a bit again today, and she ate some wet crumble mash..first time in 2 days she's shown enthusiasm for food! Keeping her inside with a companion chick until she's better (hopefully!) 
