I had a similar thing happen, I managed to talk Nettie out of poisoning me but refused her plea to kill myself if I started to turn. The conversation finished and I found myself locked in. I eventually got out by talking to her again, and the conversation went as if she had poisoned me but didn't want to give me the antidote. Luckily I managed to persuade her, and she made me promise to kill myself again, then let me out of the room.
