A few things come to mind. I'll bet that you are the type of woman that has really low blood pressure. Second, you may have issues with low blood sugar, so you need to stay on top of the eating. But, my biggest clue is that you are probably low in iron. Are you seeing a doctor or midwife? Has any labwork been done? If your labs are good, then it's just an issue of the low blood sugar combined with a low blood pressure and eating more is what is going to help - even with your nausea.
I'm so sorry. Have you looked into B6 injections or something for your nausea? I know it's hard to think about, but if you eat more often - especially protein - you may quell alot of that nausea, and the fainting feeling.
Let me know how it goes.

Let me know how it goes.