I fainted twice in my 2nd pg with dd1 - I had low bp too.
1st time was at 10wks and was very scarily carrying ds1 downstairs at the time..... I remembered being at the top and then i was laying in a heap at the bottom and he was shreiking the place down! We both got checked out and were fine, but was very scary.
2nd time I was in a massive queue in a chip shop and it was very weird. I could hear them all talking about me, and ringing an ambulance and I wanted to say "don't, i'm fine!" but I couldn't. It was like i was half concious cos I could hear them but i couldn't wake up.
Must be a common sensation. I found a man collapsed once and when I rang the ambulance I was describing him, his age etc and told them he was about 40 and the first thing he said when he woke up was "I'm only 32!" 