If you were laying on your back it could be your uterus putting pressure on a major blood vessel in your back which when that happens it slows the blood flow to your brain making you dizzy. if you aren't laying on your back, see answers above
There might be more reasons: BP too high or too low, dehydration, low blood sugar, exhaustion...
It's Very Common, I Get It All The Time. I'm Now 23 Weeks Pregnant, Been Happening Since I Was About 10 Weeks. Just Take This Slowly, Don't Get Up To Fast, Drink Plenty Of Water And Stay Cool. When You Feel Yourself Getting Dizzy, SIT DOWN. I Have Faited A Couple Of Time Because Of It. Good Luck To Bunny.