Implantation bleeding? soo confused!?

My husband and i had unprotected as late as 2 weeks ago, we're TTC #2. Yesterday my period was due, and this morning i had some brown blood so i figured i was starting my period and was all sad, then 5 hours later when i went to change the linen, i hadnt bled anymore but a few brownish red drops. I have tender and a weird crampyish, full feeling where my uterus is. Ive been tired, I know these are all signs of mentration, but wheres the rest of the period? I just took a pregnancy test and it was negative, granted its the day my period is due, at 830 at night after ive been drinking tons of fluids and my urine is probably diluted, what are your opinions? I never start off this light with a period and im stumped.