VODACOM SUBSCRIPTION INCREASE

It seems that Vodacom is using consumers to gain their profits and breach customers. I received a sms stating that my subscription was due to increase in MAY 2015. I signed a contract for 24 months with a fixed subscription but it seems that Vodacom made a Business Decision to increase the packages without valid reasoning. I went into the Vodacom Store to find an explanation, the sales lady could not even explain why this occurred and responded that \ She doesn't know and CANNOT HELP