Confusion with dates

Im a bit confused i got a positive digital test on the 3 days before my period on the 26th October saying pregnant 1-2 weeks and so i thought i was 7+4 today but i had a scan and shes dated me back to 5+4 and she couldnt see anyting in the sack as she said it was too early so is re-scanning in a week, i knew what dates i had sex last month and it was only the 14th and 16th, i did another test today and it said pregnant 2-3 weeks now how would i of got a positive so early? whats going on?