Corkysgirl -- thanks Corey!
Great minds think alike! I was also thinking the same on the Amoxitex.
I was watching her for a bit and she's acting a bit sicker than I realized. Once things calm down, she starts closing her eyes and puffing slightly.
Edited to add: ....and now she's sleeping on the floor.
Not good. So here's what I did....
I caught her and spritzed her bum with the hydrocortisone spray for cats (so it's lick-able) to knock out any itching.
Then, I buttered her bum with a light coat of antibiotic ointment. It actually did seem to help as once I applied it last night, she stopped picking at the area. Same tonight.
I tried to clean her up a bit, but it's much too irritated and very much stuck on there. It will require a bit of a soak in warm water. I don't want to stress her with that. It looks awful, but the underlying skin can still breathe, so we'll just have to leave the dried blood for now.
I switched the water. It now has apple cider vinegar, NV powder, Amoxitex, Calcium Plus and multi vitamins.
Do you think I should make up a new batch without ACV?
Still has gobs of herb salad.
Good thinking on popping a couple additional eggs into the nest. Good call! I'll do it right now.
Sent husband to the store for a new light for the heat lamp. Just got a new one, but it seems it was pre-broken for us.

So she should have her lamp back within half an hour or so.
I pulled out their veggies, including their beloved cucumbers, since they're very wet and that may prevent her from drinking enough of the medicated water.
I'm really worried about the amount of blood loss. She had a lot of blood in her feathers and as I cleaned the paper towels on the cage bottom, I noticed lots of little spots.
Any ideas of what I can give her to help with that?
She's not actively bleeding. Only when she picks at it, it seems.
I have a few bird e-collars (AKA lampshades) but they're for large birds; even the smallest wouldn't fit her. I called my friend who's a vet tech and she says the smallest ones her clinic has is for a large budgie or small cockatiel.
I know you can make them out of pipe insulation, but she's still too tiny.
I'm going to research and see if I can find a tiny finch-sized e-collar that I can have overnighted!
Otherwise, I'll need to fix what's ailing her in order to get her to quit picking at it.