What have you done to determine if the data is being used and where it's going?
If you haven't done anything, that's the first step. Optus aren't overcharging you if you are using the data without noticing.
First step is to locate the built in data usage monitor within the settings of your phone. Unfortunately it needs to be reset each month but first check where most of your data is being used and then reset it. Set a reminder in your phone to then go in and reset it at the beginning of your billing cycle each month. Resetting it today will understand what data you use for the rest of this month.
Next access the Optus App and look at the data breakdown. Again this will help determine what you're using your data on. If there's any discrepency you should then contact Optus to discuss it further.