Felt like going to pass out?

Have any of you felt like you were going to pass out during your pregnancy? If so, how far along were you? Do you know what caused it? 
This past weekend I felt fine, got a shower and started to get ready to go out when I started to feel weak like I wanted to sit or lay down and I started seeing flashes. My husband brought me some water and about 10 minutes later we got in the car to go. And as we were driving I started to feel worse, the flashes I was seeing became bigger and more frequent I was getting out even though we had the windows down and it was becoming really hard for me to talk and respond to my husband. We turned around and I checked my blood pressure it was 99/58 which is extremely low for me. I'm normally in the high 120s over 80s. I got in bed and slept for about 2.5 hits and felt better just tired. I'm 31 weeks.