The conception calculator I use gives the following for a due date of March 26:
First day of last menstrual period: June 19
Probable date of ovulation: July 3
Possible dates of conception: June 29 to July 7
Due date: March 26 (40 weeks)
An ultrasound in the eleventh week won't be off by very far, maybe 5 or 6 days if it is off at all, so it looks like you were already pregnant by July 19. Besides, your period in July had not come, and it seems like by July 19 it would have done so if you weren't pregnant.
If you would like, get a quiet DNA test done with Mr. Wrong once the baby is born just to put any doubts to rest, but I would not stress over the issue, everything points to your boyfriend being the dad.