Unwanted limit of an extra R400 per month

I have a contract with Cell C, which is actually due for upgrade at the moment!! For the last two months my R179 odd contract has shot up to R655 for last month and R611 for this month. I phoned customer care now and they told me that there is an extra limit of R400 per month on my contract, which I had no knowledge of. I took my contract out at Game Pinetown in January 2014. Thereafter while on holiday in Capetown I still went to Canal Walk Cell C store and confirmed that I do not want an extra limit on my phone and a Coloured gentleman helped me with this and said I do not have to worry about my account going over the contract cost. Since the date that I took out my contract I have not had a problem and sometimes the account would go over by less than R50 which was not an issue. I did not even check my account last month and when I checked this months account I noticed my high bills. It does not make sense to me that I took out this small contract with a l little pocket phone and pay over R600 a month. Surely I would rather have got a better contract with a better phone??!! Thank you!!!