Mine is normally very low and stays low in pregnancy. Nurses always comment on it by saying, "good!"... I know high blood pressure in pregnancy is bad news, but it'd be nice if they acknowledged the negative side effects of low, too! When I was a teenager, a doctor joked I was " barely alive"... But never said anything about it.
For those that are throwing up a lot and not keeping liquids down, dehydration can definitely lower your bp, so getting that under control should help.
I just make sure to stand up slowly and lay down with my feet raised above my heart if I feel faint. I pass out when throwing up, so I make sure I'm resting in bed with a trash can rather than hugging a toilet... I learned that the hard way after one too many bumps to the noggin on the tile floor!