Okay, Let's Go In Reverse

So, I had an ultrasound today to determine how far along I really am. With this pregnancy, it's been weird. My periods were very irregular. The last period I had was at the very end of May, but then I bled again two weeks later. And then I didn't have another period after that. So, when I found out I was pregnant, and they asked me the first day of my last menstrual period, I said the one in May. I thought the bleeding in June was breakthrough bleeding from the pill I was on, but not taking well.
But when we did the ultrasound, which was done internally, it became apparent that the baby was not 12 weeks along. And the 3 times we measured it, it came out the same...8weeks 4 days.

So turns out I am not almost 12 weeks along, as I thought, I am only 8 weeks along. Crazy huh?
The baby is due on March 26th.
So, why do I have a belly already?