Anger issues during puberty?

My 11 year old granddaughter has recently started developing and is having really bad moods. She flies off the handle for no apparent reason and has admitted to feeling very angry suddenly several times a day without knowing why. She is worried and very unhappy as to why this is happening to her. I have told her it is probably her hormones as her body is changing but that is no help at all to her and, more to the point, am I correct in this umption? Is there anything Doctors could do to help her at all? Bless her, she really needs some help.