Any idea why im dizzy?

Im 15 weeks pregnant. If I stand for more than 5 minutes, or simply take a shower, walk to the mailbox or stand in line at the store i get very dizzy. Vison goes black alot, not all the time. My body starts feeling very light, i begin to move alot, and it starts feeling really hot. 
It started before i got pregnant but has gotten SO much worse. Now its to the point where sitting makes me like this. Only laying down helps.
My iron is normal. Blood pressure and blood sugar is normal. I drink ALOT. I have a healthy diet. Im not on medications, i feel fine other than that and it goes away as soon as I lay down. 
When i asked my doctor she said "drink more water." Well i did. I even drink the juice she told me to, and it doesnt help. If my blood and everytging is normal, what could be wrong?