What Causes Cramping In The Lower Abdomen With Light Spotting?

HI , I had a missed miscarriage 2 months ago .. everything went as normal .... i had my first period and it all went smooth .... now im 6-8 days late , ive got minor cramping in my lower abdomen with light spotting .. a little red but mostly brown spotting .. ive taken 2 home tests and both came back neg ... this happened last time i was pregnant ...... im starting to feel nausiated and my breasts are starting to get tender ... what could be worng ?> could my system be that out of whack .. even tho i had my first period ?