It must have been vertigo or low blood pressure. That, or the blood loss is getting to her. I was wondering if the fake illness plus the poison AND the blood loss attributed to the stumble. But honestly, this plot made my secondhand embarrassment flare up and I closed the window after seeing her stumble and be revealed.
I stopped reading after that. I haven't returned yet, just checking the comments to see if there's somewhere I can continue without feeling shame that the MC got discovered too fast.