My period came two weeks early, heavier than usual,with a fair amount of small clots. i'm on no contraceptive; what could be the cause?

Early "period": Depending on how regular your cycles have been, heavy bleeding earlier than expected can be secondary to early miscarriage, "fibroid (benign) tumors" and potentially pregnancy outside the uterus (ectopic). Obtain a serum (quantitative) HCG test and if positive see your healthcare provider immediately. If negative, schedule an exam with your provider for an ultrasound exam and other testing.