What Causes Dizzy Spells?

I've noticed in the last 2-3 months that I've been having dizzy spells to the point of almost blacking out. I noticed a lump in my breast about 2 weeks ago that I have not had checked out yet. Could they be related? I'm 26 and had my first baby in October 2010. I was induced because of pre-eclampsia and have had days where my blood pressure was still incredibly high since then. I've checked my blood pressure after a dizzy spell a few times, but it wasn't high. I'm 5'7 and 165lbs. I've always been fairly healthy.