There are numerous other threads regarding bill increases such as yours. If you look at the contract description, you may find that after 1 year there is an increase. It is probably something that is not highlighted, not emphasized, glossed over or simply ignored during the sales pitch.
I am entering year 2 of my 2 year contract, yet my bill has increased? How has my bill gone up, if I locked in on a two year contract. No one at comcast advised on the billing increase. Makes no sense.
Your plan most likely hasn't changed, however, equipment fees, taxes and surcharges have all increased. Take a couple of bills and compare them with your current bill to see where the increase is. And always read the notes monthly on your billing statement; those will usually tell you about impending increases.