It is possible you made a recent change to your plan and the data was prorated. Let me flag a rep to help you out. 

Hi there @Anchi 

We verified the day with the charge and the charge was triggered in the first day of the bill cycle and was triggered by a rate plan change. It rarely happens, and it is recommended to reboot the device after a rate plan change. 

A credit was applied for the fee and it will update within 3-5 minutes in the Self Serve account. 

Thank you!