Rabbits do grind their teeth when happy too but only a quiet grind almost like a cat purring. The loud grinding means pain. I'd see if you can get her to the vets tomorrow especially with the mucus in her poo, it is possible the antibiotic could have messed with the good bacteria in her belly causing an imbalance. It could be because she's been on antibiotics for a while now, mention the mucus to your vets they'll be able to suggest the next course of action probably metoclopramide to make her guts move and help clear the infection out.

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