Hi, My First Day Of My Last Period Was On

Hi, my first Day of my last period was on September 1st. I had unprotected sex on September 11th. My period is Every 24-28days. So I think I was supposed to start my period on September 29th or 30th somewhere. Nothing has not came not even spotting. So around today October 2nd I am experiencing dark brown spotting of and on. Just starting the light cramping, bubbling dealing IN my lower abdomen and light lower back pain. Is it pregnancy?