i thought the same with my second pregnancy. With my first i felt great all the way through. But with my second i felt rough from day 1, i was sick, i had headaches, felt dizzy just as you do.
I also thought perhaps twins but i just got the one.
I can only assume that i was lucky to get away with it the first time :)
Make sure youre drinking enough fluids as pregnancy and the warm weather can dehydrate you causing headaches. Sickness can cause you not to eat well and that then causes dizziness too
I hope you feel better soon