It is definately possible that it is hormones. Also, she may be starting her period....the upper abdominal pain, could actually be a sign that she is getting ready to start? I believe I started my period shortly after I turned 11, and I have heard of girls starting as early as 9! I would definately sit down with her and talk, explain to her that yes, her body is changing and it will be hard. Ask her if she thinks she is acting a little out of hand, and tell her that even though she is changing, with those changes also comes more responsibilities and privilidges. The best thing is to be there for her, let her ask questions, try to come to an understanding about what behavior is acceptable and what is not, and if needed see if there is another lady she could talk with as well. It may be hard for you as her mother to let her talk with someone else, but it may also make it a lot easier for her....
Trust your instincts and hang in there!