My opinion: Sulmet is a poison and directions must be followed EXACTLY. Not a good idea for a sick chicken. Without diagnosis, antibiotics are also a waste, and excessive use when not needed have now given us resistant bacteria (will not help viruses) and may no longer be effective as they formerly would.
We are so sad when we cannot diagnose and treat. Few Vets specialize in chickens. For some problems, Exotic Bird Vets can help. And treatment for chickens is usually for flocks, not an individual bird. Commercial chicken producers just toss a sick bird. We do not!!!
Hunchy, not eating, does not tell enough. Droppings==color and consistancy? Eating or drinking if food is right before the hen? Color of comb, Easy to check for mites or lice, but not a symptom by itself. Not too hopeful, but hope anyway, that she will handle it okay.....Good luck
Crop impaction, as a check by Altair, is one we can do easily. You can feel if it is empty, or full of feed that she ate several days ago, if impacted. Possible to treat sometimes--SEARCH topic will have suggestions, including surgery that you and a helper could do.