3 days late but not pregnant?

Hi everyone.
I had my period for the last time on the night of August 6th to 7th. I always have a 28-day cycle and always get my period at the same time. Sometimes it varies by a few hours.
I had unprotected sex on August 21st and 28th. On the evening of August 21st, I got very sharp stabbing pains on the lower right side of my stomach. This lasted all evening.
I had planned to start the pill again and wanted to do this last weekend (Sunday, September 4th) once I had gotten my period.
Before I get my period, I always have the usual symptoms. Tender, hard breasts, lower abdominal pain, tiredness, headaches, etc.
I've had these symptoms for over a week now but I still haven't gotten my period!
I've also been feeling a bit nauseous from time to time for a week now, and I definitely didn't have the feeling that I might be pregnant.
I took a Clearblue test this afternoon and it clearly showed one line, so not pregnant.
I find it very strange because my cycle is always regular. Since today, I've been getting regular stomach pain/cramps. I've had them spread out throughout the day over the past week, but not like normal before I get my period.
Could it be that the test shows negative because I tested too early? I also read something about not ovulating, but why do I have all these symptoms then? And by when should I definitely get my period and take another test?
I hope for your advice/experience!