pregnant ???

The last time i had sex was the morning of january 25th and it was unprotected.. my period was supposed to come on on the 15 of this month but ive been light cramping for like 3 days and finally its very light bleeding only enough to wipe it and its a redish brown color no clots or alot of bleeding this is the second day so i decided to take a test but it came back negative i just think my hcg levels are not high enough i always have heavy periods this has never happened before . is this pregnancy or no ?