Stress..? Or normal..? Or possibly pregnant..?

Okay so I apologize, I'm new to this! About a week or so ago, I started looking for period tracking apps, trying to find a good one because I couldn't tell if my period was late. Then again I didn't track my periods before this, so I don't know exactly when my last one was. So I "guestimated" when my last period was using this app, my period still hasn't came yet. I decided to get a test, it came back negative. 😟 I still haven't started my period, its been over three weeks since my last period; I actually haven't had a period at all this month, that much I know for sure. My periods normally come every three weeks. I've been told that my period could just be late because of stress, but this late because of stress?? Should I test again..? Or when should I test again?