MTN double billing

My normal billing period was from 21st until 20th of following month. The provider MTN changed this to the 6th of each month. Subsequently they claim that they now bill pro rata instead of afterwards. Hence i got billed R2600 instead of R 1600. My contracts are fixed and should be the same every month. My query is why do I as the consumer have to pay almost double for changes that they make to their system. They claim that I have to pay extrat for the 10 days from 6 to 16. This is unacceptable as I was not billed less the previous month.