Positive then after 2 weeks negative

I got a 4 positive pt last oct 19 my last mens was sept 7 then nagpa transvaginal ultrasound ako nung nov. 4 pero sabi hnd daw ako buntis then nag pt ulit ako kahapon ng umaga negative na siya..ano po kaya possible reason bat ganun nangyari na positive then after 2 weeks nag negative.. same case po sakin pa share nmn thank you