Price Increases? On a signed contract?

I would like to understand how it is possible, that on my Smart S contract and Top up Data data bundle, which I signed into agreement with Vodacom is having a price increase? I signed these contracts based on the costing which was given to me at the time and also what fitted my pocket. How can Vodacom increase this price? Is this not a breach in contract and a breach in the customer protection act?