What Causes Persistent Headache On One Side During Pregnancy?

I have been having a headache in the left side of my head for the past week. It is behind my head and extends up into my forehead. I also have been having eye twitches that have happened in either eye. I am 6 months pregnant, and have talked to my OBGYN about the problem, but she simply writes it off as one of the "joys" of pregnancy. I am worried this is a sign of something more serious.