I think if you were that pregnant with your hormones flying everywhere, you'd also rage randomly, and probably rage a lot if you saw a sextape of your significant other with one of your best friends.
I also think she was upset because Nathan was her one and only, EVER. And she, with her hormones out of whack, flipped out and wanted to know all of the girls Nathan had ever been with, so she didn't get any more surprises. Did we all forget the nice little surprise of Haley finding out who Nathan lost his virginity to? Her freaking sister.
So yeah, she had every right to be upset. Yeah, it was exacerbated because of the hormones, but I think Haley would've been upset even without the pregnancy.