Positive PT; No Gestational Sac

Hello Mommies! FTM here. We got 4 positive PTs. Sobrang saya namin. Wala ako nararamdaman na any pregnancy symptom. Regular period ako. If pagbabasehan ang LMP, I should be in 7 weeks and 3 days pregnancy now. Pero nung nagpaTransV ako kahapon, wala pang makita even the gestational sac. I crushed! I was advised to return for repeat ultrasound. Bakit ganun? I am very sure of my LMP date kasi tinatrack ko talaga sya on the day my period started.