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.
Welcome to the Xfinity Forum and thank you for posting your question. What user: Welcome_to_Post mentioned about doing a side-by-side comparison of your most recent bills is accurate and usually the fastest way to find out specifically why your bill may have gone up. As that user mentioned, promotional periods ending, OnDemand rentals/purchases, and package price increases can also contribute to a higher than expected bill.
If you would like specifics about your account/bill, feel free to send me a private message with the following information and I can investigate further.
Thanks again for your question, we look forward to hearing from you.