She most likely has a laying type infection going on... Something like EYP or internal laying.Shes not laying and hasn't laid one since she molted in oct. I'm just asking incase she gets better and does lay. It's probably jumping to ahead.
I would not worry about egg withdrawal when it comes to medicine use.
It's probably not blackhead because they don't usually slowly waste away with blackhead. You could try the API General Cure, but I don't think it will help. If she were mine I would try a Hail Mary and treat with Baytril & tube feed her.
