Could anyone possibly help me

When i had my ultrasound scan they said i was 13 weeks witch was a shock bcause i didn't think i could be that far along! I had a normal period on the 14th but from my dates i conceived on the 9th!?🤔 then had my period after i conceived? Urgh i'm just sooo confused🤯 Any advice?😆 p.s i'm now 16 weeks💖 it still plays on mind bcause i was ttc so hard n when af came i was gutted