I have a rex rabbit who does the same thing, she quit eating her pellets and would only eat a small amount of hay. Barely would drink any water. She would do this anywhere from 2 week to a week before kindling. She just had 7 babies yesterday, lost one. This is her 3rd litter. Just an odd habit with her. Only thing I could think of is something like morning sickness.
Gritting her teeth is a sign of pain. She may have had an injury or illness you didn't notice. If she's that thin, it would be best that she not be pregnant. I know you want kits, but for healthy kits you need a healthy doe.