Change of Contract Amount without Notification

Few Years back i took a data bundle contract with cell c, 2GB for R99 over 2 years (was the standard rate at the time), after 2 years, i went into the Galleria store to cancel my contract as i did not want to pay R149 for another 24 months, when the consultant told me of the month to month option (R99 a month) instead of taking on another contract. so i would pay as i use which was super for me. last year September i noticed my debit order was R279, i assumed my husband had just used over the 2GB and we were charged for extra usage. months had passed and i did not take notice to my debit orders going out. Until January 2015 i noticed the R279 deduction again and realised its too much of a coincidence we would exceed our data limit twice. so i downloaded my statement and saw that my subscription was now R279. i called the call centre they said that the 24 month term ended and i was now paying normal tariffs. i was not contacted or asked if i wanted to be upgraded. upgrade option was R99/month for 2GB. How can Cell C deduct from me without notifying me.call centre say there's nothing they can do for me. so i am paying for something i did not agree to.i want them to refund my money.