9yr old girl got her period for the first time few mnths ago. since then it's very irregular & heavy. every 2/3 weeks. is this normal?

Not unusual: The first 6-12 mo after starting a girl will often not release eggs (ovulate) which is how the body forms the unit that makes the hormone that paces the cycles. Absent ovulation, the system is estrogen dominant and can be quite irregular. If it is too troublesome an obgyn can use a mild combination pill to regulate the cycles, but many would recommend just waiting until it self corrects.