When did your period START? That's day one of your cycle. I'm going to guess around March 23rd...
If so, it is unlikely that having sex in March would get you pregnant. Why? Because YOU wouldn't normally ovulate until between days 11 to 16 of your cycle or between April 3 and 8.
Sperm can only survive about 5 days.
So, having sex in early April it is very likely you'd get pregnant. It looks like your boyfriend, whom you had sex with in April, is the father.
Due dates and ultrasounds are estimates. They can be off by a couple of weeks.
A DNA test can confirm paternity.
Hope it helps. Good luck.