I have been having cramping with dark brown/black blood for going on three days. my period is not due for two weeks. pregnancy test said negative.. ?

Too soon to test: Conception, as defined by the generation of a fertilized ovum, occurs within day or so of the sex. However, it takes about two weeks for the embryo to make enough HCG to produce a positive pregnancy test. Blood test may be done a day after a missed period. Home pregnancy tests should be done 4-5 days after the missed period. Use first morning urine and follow instructions for the test carefully.