Could Delayed Periods With Brown Spotting Be Due To Unprotected Sex?

Hi, so recently I had unprotected sex during my period, of course the period was cut short but I m also on birth control. Now, I take my birth control at the same time every day, but I accidentally missed a day, about 3-4days after I had sex, and took the wrong day by accident. Soon as I took the pill for the wrong day I had cramps and blackish/brown discharge. I waited 2days and took a pregency test. It came out negative. So this all happened in about a 1 1/2-2 week period. I ve had this brown discharge(not the black before) before after I ve missed my period. So what exactly is going on. Is it just a late period with dried blood or something more serious? I m experiencing no soreness in the vaginal area or stinging when I wipe or use the rest room. Or any itchiness.