Oh gosh, I hope I'm absolutely wrong! I had a hen do this, and it ended up being egg parentinitis- infection from laying an egg internally. Like I said, I hope I'm wrong.
Is she still straining like to deficate or lay an egg? That is a sign, standing, no relaxing and listless and not eating/drinking. My poor girl died in a little over 24 hours, and I never thought a thing of it, but after looking in her (not expected by animal lovers) but the damage was to much and I should have put her down. Antibiotics would not help my bird, as for your own, I don't know.
Good luck !