Why I always feel dizzy as wear my new bifocal eyeglass?

[mens glasses]

last month, I purchased one new bifocal eyeglasses for computer use, because my eye doctor suggested me that the eyeglasses can both solve the vision problem of reading and distance view, but the problem is now these eyeglasses have a line in the middle of the lenses, and I can't wear it. And also, I often feel dizzy, why does it happen?