check your bank statement or credit card statement. I recently noticed that they charged me $939.08 (likely as a result of a $93.90 bill around that time, though I can't be sure), and then the next month $287.36 (even though I at that point had a massive credit as a result of the $939.08, and even though my monthly bill was far less than that). They seem to be quite willing to fix it, though the fixing process has itself not been free of errors. But if you are a T-Mobile customer, and use the EasyPay feature, you might want to see if a similar error has happened to you.
If You Use T-Mobile and Pay Your Bills Via EasyPay,