Hello. I Had Intercourse With My Partner Between The 7-9

Hello. I had intercourse with my partner between the 7-9 of October (without protection and he ejaculated inside me several times). On the 13th of October (just after midnight which would've made it the 14th, just about) I got drunk and slept with someone else but a condom was used. I fell pregnant and got a positive result on the 28th which said 2-3 weeks. My ultrasound states that I was conceived on the 13 which has got me worried. How accurate is the ultrasound? I'm worried the condom split or something and none of us realised or could it have been the deposit from the 7-9th?