What Causes Sudden Dizzy Spells Along With Nausea?

I am 33 weks pregnant, I would me moving around or walking, not sitting or laying down and all of a sudden I would get really hot and dizzy. I would also become very nauseous and I would not be able to focus on anything because my vision would start to go in and out. My next appointment is on Tuesday but I would like some information now. This has been happening off and on for a few days for the past few weeks. Any idea what it could be?