Edgars charging interest without notifying me

I've had a 6-month account card at Edgars for about 16 years. I opted for the 6-month account because it was interest free. Now suddenly interest is charged and I even got an SMS informing me that because of a previous month error, this month I'll be charged double interest. I never agreed to this and how is even legal to change the T&C's without informing customers first. If they keep this up there will be a lot of people closing their accounts, myself included.