I started having severe menstrual cramps and
heavy bleeding after my second time having
a period (at 11 yro). The heavy bleeding was caused by anemia, had to go on iron supplements which immediately helped. For the pain, I had to take birth control pills (though I wasn't sexually active). This helped for about 4 years, then the painful cramps came back but were not so severe. Everyone is different obviously, so I would suggest going to gynecologist.