Hi , I Had An Unprotected Sex On 21 November

Hi , i had an unprotected sex on 21 November ( taken the after pill in within 48h) - bleeding on 29th November, not sure about period after that but i believe it started around 11 -12th December then I had sex around 21 December , on January 25th I've done test and it was positive. 13th March I had my first scan done and it came as 13+ 3 . Is it possible that dating scan was out by 4 weeks ams i was actually 17 weeks pregnant ( from first sex ) ? All my symptoms were suggesting dating scan was accurate but I'm panicking - what if not and baby is from first sex ( where I've taken " the after pill" ). Please help!