What Causes Delayed Periods While HPT Is Negative?

My periods are pretty regular, I have cycle averaging 30 days, with a lowest of 26 and highest of 34 ever.. I got my last period on 1 Jan and I was due on 30-31 but I have yet to have my periods and today is 7th. I am sexually active so suspecting pregnancy I have done hpt but all have been negative with last one done on 5th Feb. Since 2 days I am on susten sr300 mg as prescribed by my gynac.. Just to add I had 1. Few drops of bleeding on 23rd jan 2. I went to a theme park and enjoyed lots of roller coaster rides on 27th jan 3. I had periods like symptoms for 3 days like back pain, mood swings, slight fever between 30 Jan - 2 Feb... Am I pregnant? If yes what may be the reason for negative in hpt on 36th day since my last period? If I am not pregnant then why are my periods delayed? Does susten help me?