Pregnancy test question

If I took a pregnancy test around being 3 weeks pregnant with a clear blue digital, should it have said pregnant? I'm pregnant for sure but I took a test around being 3 weeks and it said not pregnant. Then a week and a half later (at 4 weeks 5 days) it said I was pregnant. Should it have said pregnant at 3 weeks or was it too early then? Could my dates possibly be off?