Why did my hamster go vicious?

I got my hamsters a few weeks ago I got 2 female Russian dwarfs when I got them they were really friendly and very tame. I've noticed today that one of them have gone really vicious the other one is doing really well though. It's not because they're fighting because they cuddle up together all the time. Before it didn't mind when i used to wake her up to brush her but today I tried and she just started scratching the brush. And also when I was giving her a treat she was acting really vicious.whats happening to her?