Passing large blood clots five days before period. could this be an early miscarriage?

It is possible: But u should see your gyn asap or a gyn as u need an exam, blood work to r/o preg and other hormones and to check your hb level, and a pelvic ultrasound to check the uterus and ovaries, and you may possibly need an in office hysteroscopy to look at the endometrium and an endometrial biopsy and once your doc knows what the cause is then there are different treatment options.