sudden rat aggression

Question
I have a very sweet and normally quite loving and easy going female rat.  About 3 months ago, she had a litter and I decided to keep one of the females from it.  About a week ago she began acting extremely aggressive towards her daughter, walking up to her and grabbing a hold of her ears or neck while her daughter squeals and cringes.

I had hoped that it would get better when I finished making their new, larger cage, but now she will walk down 2 levels just to bite her daughter and then walk back up to the top.  She hasn't drawn blood at all, but it is very distressing to both her daughter and to me.  The poor younger girl is spending a lot of her time trying to hide in the corner closest to me, in hopes that somehow that will protect her.

It's very worrysome to me, as I love my rats very much.  I'm not sure what to do.

Answer
Hi,
 I'm sorry to have taken so long to answer this, but I've been ill. My advice is to immediately separate the two. The mother may have hormonal problems, or jealousy issues. If the problem has not resolved itself by now, then the daughter is in danger of being seriously injured or even killed. Please put them in separate cages. I wouldn't put the mother with anyone for now or maybe forever. The daughter may have a friend later on but not until her fear is over.