At age 11, I had the same problem. I remember coming home and crying many nights. It was definitely hormonal changes in my body, even though I did not start my period until 14. I also started middle school, so it was transitional. It was tough on my parents, but they just worked through it. (I was the third kid.) The most important part was that they kept my eating and sleeping schedules constant and were available everyday. Many days, they would help me with my homework to keep the communication lines open and make sure I got it done. By the next year, things smoothed out tremendously. That was really the worst year and I started focusing again after that. At that age, your view of the world changes.
Good luck! We have similar family dynamics. I had our daughter at 27 and we're having our second in a few weeks, 5 1/2 years later. It will be interesting to see what happens to our daughter at 11. :)