Yup what they said I always have low bp when pg mostly at the start (& again at the end) when the body is trying to make much more blood. I often don't drink enough and that is 99% of the time what the cause is - cant make nice thin blood without water and yes its quite possible your headache is from low bp, I get shocking migraines on occasion and its usually after spending the day running around town shopping and doing errands without stopping to drink enough (i feed the kids on the go normally & they have water bottles) when the blood is thicker its harder to keep pumping around especially up to the head where its really needed obviously, well that's my rather simplistic view of it anyway. Get yourself a big jug or even a water bottle that's just for you that way you'll be able to monitor how much fluid you're actually drinking. I would try aim for at least 3-4litres a day while pg especially whilst you're still BFing also.
Hope you get some relief soon sweet