I've fainted probably 6 times since getting pregnant. After I hit my head the most recent time and having to go to the ER, I talked to my OB about it. She said to make sure I drink lots of water, have some sugar or caffeine (both can increase blood flow). Otherwise, lay down and try to prevent any injuries (i.e. hitting your head, falling on your abdomen, etc). She said I could also try rubbing my carotid artery when I feel faint, as that might help draw blood up toward your head.
Bottom line, she didn't seem all that concerned about it except for the falling part. Just call your OB and let them know what happened. They'll likely just make a note in your chart.