It may be just her way of releasing frustration when she hasn't learned how to express her feelings properly yet. It must be hard for you to watch, but I would say to try to completely ignore her after she's done it, otherwise she will see she is getting attention for it and continue doing it.

Is there anything going on that links the last time she did this, to this time? Anything from a stress/upheaval to a simple thing like a change in routine?

If the self biting is after being punished, try this: http://www.drspock.com/faq/0,1511,1785,00.html

Not sure if it will help at all, but I got this off t'internet:

What can you do to help prevent biting?

- Try spending extra one-on-one time with your child if you think he is trying to get attention. (Not immediately following an act of self-biting.)
- Play together actively, games that might help the child burn off the steam that might otherwise be kept inside and come out later as a biting incident.
- Interrupt the biting or hitting incident. Pick your child up hold him in your lap. Tell your child as best you can that you won't let him bite himself because you love him and it's your job to take care of him.
- Try to change the situation and redirect the child's attention to something else.
- If possible, try to watch for this behaviour and attempt to circumvent it before it happens.
- Teach your child acceptable alternatives. Perhaps, choose a keyword that word help your child express when he's upset. (Or bang on some drums, or shake a noisy toy really loud, or even bite a cushion or other safe biteable object)
- Praise your child for appropriate behaviour.

What not to do?

- DON'T give your child extra attention or act overly concerned about the bite. Otherwise, self-biting may become a regular behavior anytime the child doesn't get what he wants.
- Check your means of punishment. Make sure that your form of punishment isn't too harsh or frequent for your child. Your child may be getting too angry about the punishment, unable to think about what he did wrong, and unable to express it.

Let us know how it goes. xxx