INCREASING MY SET LIMIT WITHOUT PERMISSION

I had taken out a tablet & data contract with Cell C in December 2015. cost of the contract was R125, this included 500 MB anytime data and 500MB nite data. I was told that i could set a limit on the contract as my account would be billed once the allocated data was used. I set a limit of R50 as this was the minimum amount that it could be set at. I for one don't take notice of my debits as i take it that i know whats meant to go off each month. Lat night while using my sons tablet i checked, out of curiosity, how much data he had left for the month, and to my absolute shock i see that the usage for the month is at R312. how could this even be possible if i have a set limit of R50???? i call the customer care line today to query this , to be told that they took liberty at doing a credit rating for me and increased my limit to R609!!!!!!!! WHO GAVE YOU PERMISSION TO INCREASE MY LIMIT! WHY DO YOU TAKE IT UPON YOURSELF TO INCREASE MY LIMIT WITHOUT MY KNOWLEDGE???? TO SAY I AM ANGRY WOULD BE AN UNDERSTATEMENT.... i am totally disgusted with You Cell C!!!!! I have been a customer of yours for many many years. how can you treat your customers like this. this is ****************!!!! please show me where in my contract i had agreed to such??? ******* ******* *******