Sex 4 days before expected period... Now 5 days late??

Alright so I am a bit confused. My cycles are average 28-33 days with my period lasting 7-9 days. They have NEVER gone past 33 day cycles. My last period was October 23 and lasted 8-9 days. Which means NO later than November 25th (would he cd1) I should have started. I've tried opk's before and most read positive for 4 days starting on cd18-21, but I have also had another 5 days of positives the week of my period being due... So I stopped taking them because it was confusing and stressful to not know which one to go by.

This month, I only had sex once and it was on the 21st.... Period due no later than the 25th.. Now it's the 29th and still no period, spotting, cramping, no sign of period coming, etc. Could I have gotten pregnant 4 days before my expected period and it caused it to not come???