Could it be?

I am not TTC but also not preventing it. We had intercourse 3 days prior to ovulation this month, so I assumed there was hardly a chance... but I am 6 days late, I've taken 3 tests, once on Saturday, one Monday and one today, all negative. My cycles are usually very on track. Exactly 28 days. I'm on CD 34 now still no sign of AF. I've been extremely tired, horrible headaches the past 2 days and having nausea. Yet, no sign of even a faint line. What's going on?! Ahh! Help!