Help please

I started having cramps and they come and go but when I do I have dark reddish brown spotting it started Saturday and at first thought implantation but it would of only been 4 days past ovulation period it it starts at the end of ovulating if it starts at ur most fertile day that would be 6 days does anyone know what it could be or when the dpo starts?? And it was weird cuz my periods are always four to five days but my last one started 3 days earlier than normal and lasted 12 days