My niece started her period at this age, 8. She has very heavy periods and very bad cramps. When I say heavy, I mean she's going through an overnight pad every thirty mins or sooner.
When she was seen by a Dr, they did lots of tests, scans, blood work, x-rays, you name it. They said that her bones are that of a teenage girl. They asked why she had not been seen for this sooner. I'm assuming that I didn't get the entire conversation that took place and that there must have been other symptoms that indicated an abnormal change for her age, since they took her to the Dr pretty quickly following the aggressive initial cycle.
The Dr want her on depo, but not depo every 3 months. Depo every 28 days.
I'm not sure if just having her cycle this early is cause for alarm, since my niece is a unique situation, but I would talk/take her to her Dr to be on the safe side.
Hope this helps, and good luck.