There are many reasons why you become dizzy when practicing rolls. Several others have hit on the one main reason. The semicircular canals in your ears help you to maintain balance and sense movement. However, your eyes are also important factors in the determination of movement. Here's an example. Have you ever noticed that while sitting in a car at a stoplight, and the car next to you begins to move forward slowly; you get the "sensation" of moving backward? This is because your eyes play a little motion trick on you, the semicircular canals in your ears aren't receiving any motion input, but your eyes are.
To help alleviate the dizziness, you need to get your eyes and ears to experience the same motion inputs. In other words, keep your eyes open while you roll. This will help coordinate the motion inputs of the semicircular canals and the visual inputs of your eyes.
Also, the more you roll, the less dizzy you become. Eventually, you'll get used to it. have a good day!