Sudden price increases usually signal the end of a promotional period, which can be written into a contract. If you go back over past bills, you'll most likely see a warning of it somewhere on the bill. Have you lined up an old and new bill, and gone through them line by line to see where the increase comes in? That should show you what happened.
Hi, cjeckery - Welcome_to_Post provided a great explanation of what could happen with your bill. If you still have any questions, please send me a private message and include your full name, the account holder's name and address as it's listed on the billing statement, a phone or an account number. Thank you!