I need advice, I'm really confused and a little scared.

So, I'm pregnant. I got my positive July 27th. My husband and I had sex June 29th, July 2nd and July 17th. Going by June 29th and July 2nd, I should be around 6-7 weeks. Well, I got my blood test last Monday and it says I'm 4-5 weeks and I got my ultrasound yesterday and it says I'm around 5 weeks. July 17th would be the only date that works because I'd be 5 weeks and 2 days. But that doesnt make any sense because that would mean I got a positive in 10 days? is that even possible or should I prepare for the worst?