Have you changed your monthly plan? Then your next invoice/bill may be slightly higher than expected. You will be charged for ...
You pay for your old monthly plan until your new monthly plan starts. From then on you pay for your new monthly plan. You will therefore see two short periods just on this one invoice/bill:
You also pay for the coming month in advance, so you will see a charge for the full monthly price of your new monthly plan.
You already paid for this month of your old monthly plan on your previous invoice/bill. Naturally you will get that full amount back.
Did your usage go beyond your monthly plan? Those charges will appear on your next invoice/bill.
Tip: Also take a look at the things you should keep in mind when you switch to another monthly plan.