I have them, too, though not quite as often as you describe. I know they can be a symptom of pre-eclampsia or dangerously high blood pressure, but mine is always low to abnormally-low. My dr. said they are caused by fluctuations in bp, particularly when you stand up/sit down/turn head. Could also be caused by increased blood flow. Not a great answer for sure, but my OB was not concerned about them and I'm very high-risk so I'm being monitored closely.