You will be able to see any changes in your contract in the online bills. If this charge has just happened you should be able to compare the most recent bill with the previous one to check what has changed.
It may be that you have come to the end of a discounted period. The dates for these terminating are available on each bill, and on your contract which is available to view within My Virgin Media
The Service you do for others is the rent you pay for your room here on Earth - Muhammad Ali