Severe headache!

I'm almost 17 weeks now and the last 3 days I have had horrible, horrible headaches. I cant function, and my eyes water nonstop bc of the pain, I have a blood pressure monitor at home and it reads low when I have the headaches (though I'm not sure just how accurate it is, it's quite old, I got it from my FIL) when I lay down the headache seems to die down a bit, but even just sitting up in bed brings it back full force :( any suggestions?