These symptoms are something that need to be evaluated when they occur. It could be dehydration, but it could be something much more serious. Forget the doctor who is ignoring your concerns, and go directly to the ER. When you're there, ask if they can recommend a doctor who isn't a complete idiot. Good luck.
Be minful of preclamsia and toximia. These sound like those symptoms. Good luck :-)
I've had the same thing my entire pregnancy. When you start to get light headed lay down on you left side with an ice pack on your head. It keeps me from blacking out and fainting. Plus them if you do faint your not gonna go far since your already laying down. Also, try to eat more proteins through out the day, they'll keep your blood sugar at a more stable level which also could help. Your dr should test your blood pressure the symptoms you listed are all signs of high blood pressure which is very common and not something you did wrong. Good luck :-)