Wow, I am astounded at how many women responded to get her tested for bipolar disorder. This does not sound like the classic symptoms of bipolar disorder in the least. Bipolar mood swings generally last several days to weeks to months and are not normally daily changes. They are also very extreme. When in the manic stage they can be practically bouncing off the walls, euphoric and believe they are invincible whereas the depression cycle is extreme depression whereby they sometimes can't stop crying, can't get out of bed, have an extreme feeling of hopelessness.
It does sound like hormones perhaps or even just frustrations and confusion over body changes. She could also be upset about something but not mature enough yet to really get a handle on what it is and how to express it.
Girls can start having normal body changes and hormone changes as early as 8 years old. I found that there are alot of great books on amazon.com that are written both for the girls going through the changes as well as for the parents. Also, it might be a good idea to talk with her and tell her that you've noticed she seems a little sensitive lately and ask her if anything is wrong or if she wants to talk about it. Its a good idea to get your child to open up to you about their feelings and it lets them know that you believe that their feelings actually matter.
She may be displacing her feelings and she may be frustrated or upset about other things entirely. She may not even know what is upsetting her but if you ask her a few questions like "is something bothering you at school" or "did something happen with a friend" or even "did I do something to upset you," she may begin to think about what might be bothering her. I have done the same with my daughter and if she says she doesn't know why she's acted a certain way, I ask her to think about how she is feeling and see if she can figure it out. Let her know you're always available to talk if she needs you.