headache and vision disturbance

I'm 36 w & 3 d pregnant. For the past 4 days in a row I have had a really bad headache accompanied by flashing floating light (the light you see after you stared at a light or the sun and looked away) I went to a medical center to see what can be happening and they drew blood and got a urine sample but it came back normal. Tomorrow I see my Ob. But does any one have a clue on what might be causing this? I can't take it anymore it's so painful-