Hi, I Had Unprotected Sex On The 11th Of September.

Hi, I had unprotected sex on the 11th of September. My period usually comes on the 18th but is known to be irregular. Tonight for the first time I have noticed little specks of blood coming from out of me, it is really light. Today is the 25 of September and I have had no period yet (unless I am starting it now?) I have had in and out cramps and a sharp pain one time in my back, I'm pretty weepy (but generally I have always been emotional). I took a pregnancy test in the beginning of this week which was probably too soon but it came out negative. Could this be a sign of pregnancy ?