i have unlimited calls/text/data and £0 cap but bill is significantly more than usual?

I pay £62.99 a month for a iPhone 14 pro max with unlimited calls/text/data and a £0 cap limit so it’s impossible to go over, I got the phone in October with a £150 upfront payment, I paid my bill in November for £62.99 and everything was fine but I have just received an email saying I owe £81.62 but there had been no missed payments or anything as I pay via direct debit and there is unlimited everything in the contract and not to mention I can’t go over if I could as I have a £0 limit for the cap, how is my bill almost £20 more? It’s only the second month I’ve had the contract?? Not to mention christmas it round the corner, it’s a joke the breakdown of the bill doesn’t explain where the extra £20 comes from either, if this is just an issue then I need it resolving