Could I be 16 weeks pregnant instead of the estimated 5 weeks!?

I'm terribly irregular and my last batch of periods lasted over two sometimes three weeks, then skip a month and show up whenever the next. My last one started mid january and lasted till feb 6. Then mid march after having very rough sex i bled for two days directly after intercourse. And no AF yet, positive at the doctor. I didn't tell her that the two days were directly after intercourse so of course she s going to assume. I'm just really trying to figure out my EDD. Could I really go 4 months not knowing?! I'm making an ultra sound appt this week