Many things can make a period late, stress is one of them. I genuinely wouldn't start worrying about it being late unless a. you show symptoms or b. its more that 2-3 weeks late (at which point you can check with a regular test easily). However stressing wont help you, the best thing you can do is wait for a bit longer unfortunately.
In addition, while not as reliable as regular pregnancy test most early pregnancy tests are accurate 80% of the time, if it says negative, its likely you are.