2-3 weeks late

So i have to <a href="https://play.google.com/store/apps/details?id=com.glow.android">ovulation apps</a> one say that i am 3 weeks late the other say 2 weeks. I went to the Er about a week ago for unrelated matters. They ran a blood test it was negative, yet im still playing the waiting game, could the hospital have made a mistake?