Unfortunately, the more attention you give in the matter the more she's apt to do it. It's something new and she's going to do it. Now if she's teething, maybe give her a teething toy to chew on to distract her from the grinding. Sorry can't be of more help, it's a stage they all tend to go through. Next she'll figure out something else like blinking and squinting her eyes makes things look different. Good Luck!
Does she do it when she's sleeping or awake? If she does it when she's awake, try to dristract her by giving her a cold toy, a teether, a cold wet washcloth, frozen fruit, etc. If she's only doing it while she's sleeping, there isn't much you can do. Its pretty normal and most kids grow out of it--I never have :~)