Increase of Contract

My fiance received an e-mail confirming that the contract that was renewed in November 2018 will be increased with another 5%. I would like to know if this is legal because as far as I know you do an renewal and sign a contract for the next 12 months at a set amount for the next 12 or so month. Now that you query the increase you get told that if you don't want to pay you need the pay for the outstanding contract which is 18 months. No calls to explain just an e-mail.