Hi Cass - I naturally have low BP too! One of the first ways I could tell I was pg was with the fainting/headaches - so classic I felt like I was out of one of those 1950's soapies :-)

Drink LOTS of water - it helps with the nausea too. My SIL told me about it I think, she explained it as diluting the hormones in your blood - I don't know if it actually works that way but it helped me a lot! With DS when the fainting/nausea was really bad I would suck on lifesavers which worked too - peppermint ones

Hope you are feeling better