No, you will be billed at the rate that was valid when you used the gas. The usage is charged in arrears. If you bill was due on the 17th, then this will cover usage up to a few days before this.
In the next bill you will be charged at both the old and new rates I believe. What they usually do in these situations is estimate the amount of gas used up to the changeover date, charge this at the old rate, then charge the remainder at the new rate.
Leo