What Causes Vertigo During Pregnancy?

I have been having episodes of vertigo/dizzy spells for the past 2 years. The first one started 7 months before I became pregnant with my son, and continued on throughout my pregnancy. I am now 26 weeks pregnant with my 2nd child and am still having these episodes. I am also suffering from weight loss, I struggle to hold onto every pound I have. Pre-pregnancy weight with my 2nd child was 98lbs and at 26 weeks along I am now 110lbs. My vertigo/dizzy spells make it impossible to walk. I have to grab hold of something near, bend over, and shut my eyes, and wait for things to quit spinning and my head to quit pounding before I open my eyes back up. This doesn't seem normal to me, esp. with it going on this long. Please help!