If you wear your sunglasses for the first time, it is normal that you feel dizzy because it usually takes some time to get used to them. If you've got used to your sunglasses, your dizziness may caused by the sunglasses itself. Maybe the temples are too tight or the nose pads are not in the right place, which cause pressure on you and then you feel dizzy.
It is quite an odd problem. You can try to wear polarized sunglasses to see if you still feel dizzy. I have seen people also feel dizzy when wearing regular sunglasses but feel good with polarized sunglasses.