I need help ladies..

So yesterday we managed to visit an OB in spite of lockdown. I really thought I'm 8 weeks pregnant and ready to hear the heart beat but when the OB ultrasound me it's still a sac and I'm just 4weeks pregnant. I'm really puzzled right now cos my last first day of period was February 16-20 so glow app says here that I'm almost 9weeks? But my OB says I'm only 4weeks that I got pregnant March in my next cycle that I don't have my period but got pregnant so I'm really puzzled if not February 16 is the right date to indicate to correct my calendar then what day I should choose to set it 4weeks today? Cos can't edit my glow calendar now. Thank you for helping hope you get what I mean..