What Causes Light-headed Feeling And Dizziness After Increasing Dosage Of Zoloft?

I upped my dosage of zoloft from 100 to 150 and am feeling very lightheaded and dizzy, almost to the point where my vision fades and I feel like I am going to black/pass out, but I never do. I have been on the 150 for 2 weeks now. Is this normal and will it eventually subside like just when I started the zoloft? Also, how many more weeks do you think I should expect the anxiety?