Is a little more than 1 week (10 days) too early to determine pregnancy?

After the protected incident (fri), I took a plan B within the 72 hours (mon), and waited for a week since taking the pills. I felt nausea the day after and minor stomach cramps. I have not seen any noticeable spotting and this has caused much concern because its been more than a week. My period is normally due for the 17th (in about 5 days, so almost two weeks after taking the pill) but im nervous all in all. I just took a pregnancy test (ept certainty, determines 5 days before missed period) today, and it declared "not pregnant" within a minute (packaging says 3 min). Two questions: Is it too early? Is the fact that it said not pregnant in a min, mean its not accurate? Please help. thank you <3