Reason being, while both meds are SSRIs, the specific and intricate
mechanism of action of each med varies quite a bit. Therefore, you could be experiencing a form of a discontinuation syndrome (fancy term for a w/d like situation). The good news is, introducing the Zoloft WILL help to minimize the severity of those symptoms.
Again, your dizziness could be caused by several different things, no one can say for sure, but yes, you are correct that your theory is a possibility.