Period late but not pregnant?

I ovulated on and around the 18th of October. Then I had unprotected sex on the 21st of October. I bled for about 24 hours on October 24th (medium flow, dark colored with hints of red) then spotted the 25th until the 27th but only when i used the rest room, not tampon or pad worthy.

I've been feeling bloated, gassy and constipated, with lower back pains and some minor cramps off and on since the 25th. I had blood work done at the doctor but it came back negative. After speaking with the Dr. he said since I had sex one week before my period was due, if i did indeed get pregnant it was way to early to test and to wait 2 weeks and see if i get my next period.

But I'm still wondering, how could my period be late if I'm not pregnant? What are the chances I may be pregnant? My period was due on October 26th or 27th and I still have yet to see a normal 4-day flow like I usually have.