Is there an explanation as to why I have fainting spells a few times a week during this pregnancy?

Time after time I have had episodes of dizziness, near fainting, and fainting. Every time but once it has happened while at work. Today I blacked out all the way and fell to the ground at work. I was taken to the hospital and they ran tests and bloodwork. All the labs were normal. Again. This keeps happening and I feel like the dr’s assume I just don’t eat enough because I’m slender. Let me make it very clear to my fluther friends, I eat plenty. I can out eat my boyfriend. When these fainting spells happen, I have just eaten usually less than an hour beforehand. Example: today I ate a bowl of oatmeal and a glass of orange juice less than 30 minutes before I fainted. Some dr’s also suggest maybe I’m just going from one position to another too quickly. Also not the case. I am never changing positions when this happens. I was standing today at a cash register and very suddenly I felt myself going down. During my sonogram a couple weeks ago I fainted on the exam table. I had been laying flat on my back when I blacked out. Not standing, or changing positions. Yet, the dr’s still insist it’s either lack of eating or changing positions. What do I do at this point? Keep passing out at my job? I fell hard this time and hit my spine on a ledge. Luckily it’s just a bruise but imagine how much worse it could have been. I’m so frustrated. How many more doctors should I see before giving up? One nurse at the ER suggested that due to my very small frame the baby has little room and sometimes blocks the main artery that pumps blood to my heart, causing me to faint. She said it could get worse as the baby grows larger. I’m only 19 weeks. I might end up going out on disability. My job requires 9 hours constant standing. Anyone have any ideas as to why this is happening?