Whatever data you don't use will rollover to next billing period, but if you don't use it, you lose it.
I am sure that you use your allotment first, then any rollover left.
Say you have the 10GB plan, and you use 5 GB, the next billing period you have the 10 GB plus the 5gb rollover. If you use 11 GB, you don't pay any overage, but then you don't have anything to rollover because the rollover is only good for 1 billing period.