@Belsingh -- did you have some "credit" listed on the invoice that was due on September 16, that reduced your monthly payment to only $16 ? It could have been a one-time credit, or your "two-year Value Plan" (with monthly promotional discounts) has expired, thus ending your "promotional" rate.
Can you logon to the My Shaw Portal, and view all the "Transactions" for the last 24 months, to see, in detail, every charge & credit made to your account?