I had a lot of dizziness with my 2 previous pregnancies. I've had little waves of and on this time, but nothing too bad yet. The other two times got so bad that I didn't dare shower if I was home by myself - something about the warm water made it even worse and I came really close to passing out many times.
I don't know what causes it, I think a lot of my issue was that I couldn't eat and I was dehydrated (I had hyperemesis). I wish I had some suggestions for you! The only thing I can think of is making sure you stay hydrated.