Unfairly billed

I have been a loyal contract Cell C customer for the last 12 years but I am currently in process of cancelling my contract. I renewed my contract 2 years ago to the Samsung S4 with the new contract having 100 minutes, 100 sms and 1 gig data. I recently received a bill of more than R1200 rand for the month of July having not exceeded any of the 100 minutes, sms or 1gig data. When I contacted Cell C they informed me that my contract ended in May and the contract continued in June at the same cost without the 1gig data and that I was being charged for data usage in June. This is completely unfair as Cell C renewed the contract but excluded the data and then charged me for data usage. They did not contact me to inform me that my contract was being renewed or that my data usage was being changed.