Hello,
Such heavy intermenstrual bleeding can be due to can be hormonal imbalance(can be due to PCOS),hypothyroidism, cigarette smoking or underlying Chlamydia infection while on birth control, ectopic pregnancy, miscarriage after pregnancy and fibroids and polyps(rare in teenagers).
Seek a gynaecologist immediately who will take a careful medical history ,a physical examination, including a pelvic examination.
I hope it helps. Take care and regards.
it could be a misscarriage thou you wud of had cramps and clots in the bleeding. see your gp for expert advice