brown discharge in place of my period after 3 weeks of unprotected sex

my cycles are all normal. Last one was 1-26-13 (lasted 5 days) and then I had unprotected sex 2-4-13(he pulled out) *But yes, im FULLY aware of precum* (today makes 3 weeks since I had sex). My new cycles expected date was Yesterday(2-24-13) before then, I experienced nausea about 2-3 times and Slept like a Hibernating bear! took a Pregnancy test just about 11 days after sex and had negative results. About 2-3 days ago I began experiencing my usual "Before Period" Symptoms; from breast tenderness to Slight cramping. yesterday, Late afternoon I got a HORRIFYING headache. Ate something and it disappeared. shorty after I went to the bathroom And there was a small brown Stain On my underwear. I placed a pad thinking "my periods finally here"(it's always ON POINT). I go check a few hours later, and my pad is Still White! Nothing on it, so I removed the pad. this morning I go Pee and there's brown discharge again! being I have normal Periods, I know exactly how mine begin and End! this isn't normal FOR ME. my periods start bright red until about 3 1/2-4 days later, then it'll start dying down. Anyone else ever experienced something similar? what were the results ? Thanks in advance! :)