What can cause dizziness upon standing?

Everytime I get up from sitting or laying I get horribly dizzy. Also my legs fall asleep extremely easily mostly my right leg but this has been happening forever but the dizzyness has been happening alot more recently. The only connection I can make is that i started to watch my calorie intake when this started to get worse. This is causing headaches (which I've had mostly everyday since I was a kid), and making me feel a little lightheaded all day. Also it's causing me to get an amplified headache when I smoke weed when smoking normally prevents and deaden my headaches. Please don't answer if you plan on saying stupid crap,serious answers only.