Check the type of lighting sources you are in. I got terrible headaches with both of my pregnancies (which is actually not that uncommon) and would lose my vision periodically with my second. After a complete workup that revealed no problems, my doctor suggested that I not work in flourescent lighting - so I got soft-light desk lamps for work and turned off the overhead lights (he actually advised using a visor but realistically I could not do that at work...in a suit!). For me it was definitely triggered at least partially by light and I tried to use only natural or very soft light whenever possible. This made a big difference and I never had problems losing my vision after that (which is a type of migraine), although I would sometimes get a headache (but not as bad).
Also, did you drink or ingest lots of caffeine prior to being pregnant? You might be going through caffeine withdrawal. If you are, you should talk to your doctor about putting some caffeine back in your diet safely. Even if you never took caffeine before, this may help you. But check with your doctor because there is lots of controversy with caffeine and pregnancy.