Out of interest HF what do those symptoms mean?
Well one common disease which can suddently strike at maturity is Lymphoid Leukosis. A rather nasty and usually fatal disease which is passed vertically through the egg and has a 16 to 24 (typically) week incubation period. One symptom is as you describe others are emancipation and depression which don't fit your hen so fortunately it doesn't look like this is likely.
You can google for LL and check it out but don't become a hyperchondriac otherwise you'll start thinking it's every disease under the sun. The good news is that she doesn't appear to be depressed and is still eating and drinking well. Did this condition start after the a/b treatment? It could possibly be connected. Killing off the normal gut microfauna will allow it to be repopulated possibly by harmful ones. Probiotics are worth considering after a treatment of a/bs as at least in theory they encourage repopulation with helpful bacteria. I don't ascribe to feeding yogurt as a regular thing though but there are lactose free probiotics which might be worth a try.
Other possibilities are a kidney infection or possibly if she's free ranging she may be picking up a toxin from the compost heap or similar. Is she laying?
Could be worth consulting your vet again it it continues.
HF