oh no!
i hope she recovers quickly and it doesn't happen again.
i had a litter of 11 kittens one time and i seperated the boys at 4 weeks into their own cage and on the first night i heard a terrible screaming - they had ganged up on one of their brothers (a gorgeous black eyed white with huge ears - my secret favourite) and by the time i had got to the cage they had bitten his ears down to little stumps and bitten off the toes on his back feet
i put him in his own tank straight away and he recovered well, although he never looked the same. He had a wire lid on his tank and he loved climbing across it but because he had no back toes to grip with he would just hang off his front paws (like he was on the monkey bars) it was very cute and acrobatic to watch - he was still my favourite - i called him spike.
it didnt take long to figure out which one in the litter started the fight (as by the time he was 6 weeks old he couldnt bare to be around any other mice and got very nasty towards them) he had to live alone as well
you might have a little bully in your litter so if you do be carefull he doesn't turn on his litter mates too
best of luck - i hope it turns out well
