First, there's a post at the top that acts as a sort of guideline of what information we need to get a good picture of what's going on with your bird so that we all can better help you.
It's here:
https://www.backyardchickens.com/forum/viewtopic.php?id=3569 If there's informaiton there you can provide, it would be accepted gratefully!
I'd like more information on the antibiotic, please. What's its name, how long have you been giving it, why did you give it, etc? Incidentally, please avoid using antibiotics until you're sure there's a bacterial infection. This doesn't necessarily sound like one. Antibiotics are very rough on birds' systems and are often overused unfortunately due to industrial practices. So it's best to wait before medicating. It's not your fault - this is the common advice of most feedstores. "If you don't know, give them this packet of _______". It' not great advice, unfortunately. But you're giving them now so you **must** give them at full strength for the full number of days. Otherwise you risk that antibiotic not being useful to you again.
OK, the bird... have you considered gapeworm? Is she making a choking or coughing noise, acting like she's trying to dislodge something from her throat? Is she an adult bird?
In the mean time, you'll need to jump start her nutrition and hydration. Separate her if you must. make a mash of her pellets (ground down in the blender), some water (pedialyte - generic is preferable - if she's not been drinking), part of a boiled egg yolk (freeze the rest for later use), a dab of plain yogurt, a dash of honey for some energy - or of karo syrup. Mix into a moderately moist but not very wet mixture, see if she'll eat it. If she doesn't, take a portion of this and mix with more pedialyte. Dribble into the side of her moutha few times of day. This hopefully will jump start her energy and increase her appetite.
Glenda Heywood on here has a recipe for a mash including vitamins that is a very good way of putting vigor back into an ailing bird. I'd highly recommend that you look it up, or PM me and I'll get it for you. It has a few things mine doesn't, a little more pumped up.
In the mean time, can you give us whatever info is on the post I've provided above (especially description of her droppings which will tell us her nutritive state at the moment) - and we'll do our best. Thank you!