If her crop feels like a water balloon then that suggests there is a blockage somewhere, as water should filter through her system fairly easily, not stay in the crop. I would hold off giving her antibiotics orally until you know her crop is working, other wise it will just sit in her crop with whatever else is in there and not be absorbed. The fact that she has not pooped also points towards a blockage. That and the breathing difficulty are often linked to a problem with the reproductive system, particularly with an older hen.
When you say her rear sags, do you mean below her vent or between her legs and does it feel loose and saggy or full and hanging or a bulge? Has she been walking strangely recently.... perhaps with her legs wider apart?
How does her breast feel? Is her breast bone sharp under the skin or is it reasonably covered..... compare to one of your other hens of a similar breed, so you have an idea of what healthy feels like.
I would not give her any more fluids until some of those in her crop have filtered down into her system. If you can, give her a heat source as she will be struggling to maintain her body temperature whilst she is unable to eat. A heat pad would be ideal. Sometimes warming a bird up can really revive thme significantly, although it will not cure them, but can give them the strength and will to fight whatever it is.
The chances are she has been sick for some times and has been hiding it, so she may now be low on body stores. Once you have her warmed up, I would sit her on your lap on a towel and gently massage her crop, particularly the lowest part.
If you have some Vet RX or Vick's Vapour Rub, put a little on her comb and wattles and chest, to see if that will help open her airways. I would do that before you massage her. If the massaging distresses her either be more gentle or stop as any agitation will make her breathing problem worse.
Get back to us with any further information you can regarding body condition and that saggy area.
Good luck with her
Barbara

