The baby is 100% my boyfriends right?

In December 2015 me and my boyfriend broke up. In February I had sex with a guy twice on separate days but only once he finished inside me. I found out he had a girlfriend so I stopped talking to him. Had periods since then. My last period started 29th April and ended 5th may. On the 6th may I had a pelvic ultrasound because I had been having pain in my ovaries but scan showed everything was normal. and also in May I got back with my boyfriend and had sex on the 12th. That was the first time I had slept with someone since the guy in February and the first time I had sex that whole month. So I know that's the only day I could have concieved. On the 3rd June, 3 weeks after I had sex I got a positive test. When it came to my first ultrasound baby was measuring the same as what my last period said. But I thought it should have been under that? Is the baby 100% my boyfriends? I don't know why I'm so paranoid about this. The dates on the scan got me confused I thought the last period dates were just an estimate