Feeling dizzy when rolling can be quite common because, unless you're a ninja, it's something our bodies don't do on a regular basis. Moving between the forward and backward roll is especially disorientating for the body, but it's something that you'll get used to with practice.
Here's some way you can better acclimate:
1. Put a slightly longer pause after your forward roll and into your backward roll.
2. Reduce the number of repetitions per set until you're ready to build up to bigger sets.
3. Put some extra rest between each repetition to lessen the dizziness.