Heavy bleeding & sore back...

Yesterday afternoon I began lightly spotting and slightly cramping. I'm a little thrown off because my period isn't suppose to be here for a little over a week still. I went to bed comfortable & without spotting but at 2:30 this morning I woke up cramping horribly and realized I was bleeding a lot heavier with a few clots. As of yesterday I'd be 6dpo and believed not to be pregnant at this time.

Could my period just come over a week early? Is this implantation bleeding? Any advice as to what could be happening??

Has this happened to anyone else?