What Causes Dizziness Along With Hot Flashes?

I m 39 weeks pregnant as of today. Starting yesterday, I have had spells were I feel light headed and then once that subsides I get hot flashes. I checked my blood pressure and it was normal, 118/73. The feeling passes within 20 minutes or so. I have normal fetal movement daily. Also for the last two days I have had loose BM s more than one a day and constant back pain, but no abdomen cramping. Rare abdomen tightening throughout the day. Is labor nearing? I have an appointment tomorrow with my OB.