Most likely concieved after july 19th, time to book in with GP/midwife to find out how far along you are and they can answer all your questions properly xx
It can happen , I had my period 10 weeks pp and then found out I was pregnant couple months ago , so you could either conceived before or after ,I had my little girl and boy have 10 months between them xx
I would just consult your GP about getting an HCG reading and dating scan.