over charged on upgrade first month

So my contract was up on the 5th of this month. A couple of days before it ended I used the app to upgrade to a 6 pound a month sim only deal. I was expecting a 6 pound bill but its the same as the last 24 payments of 34 pounds. Also 3 pound was charged to be taken also on another separate bill after the upgrade. So for this my 25th month on id i will be out of pocket over 37 pounds not the expected 6, which makes no sense. All previous  payments have been completed and paid for my old contract im sure. My question is why am I being charged a 25th payment for a 24 month contract that I upgraded from. I was expecting a simple payment of 6 pounds. I don't like unexpected over charging so I cancelled my direct debit for the meantime. If its a legit mistake then please correct it.  If I am misunderstanding something and im not being charged for a extra month could you please explain it to me, thank you.