Price increase nonsense

How is it possible that the price can increase of my contract after I signed and agreed to the current price that I am paying. I never gave autherization for a price increase. I have been with vodacom for 10 years with 4 of them being contracts on my own name. I currently have 2 contracts and I have no problem cancelling both and go to a different service provider. And buying data that does not even last a full month or cary over when buying data again is *****nse. Why am I paying for something if I'm just loosing it. I'm slowely getting tired of the service vodacom is providing....