Unhappy Faithful customer of 3 years

I recently purchased a 3 gig data bundle from CellC through my cell phone banking. As usual once I complete the transaction I get a SMS detailing the expiry of the bundle. There is no notification on expiry during the sale and only after the sale, I receive notification on expiry. This did not concern me as I had always purchased data bundles this way in the past and I received notification SMS that the bundle will expire a year from purchase. This time when I bought the 3gig bundle I received a SMS reflecting that my bundle will expire in 30 days. This was shocking!!!! I immediately phoned CellC to inquire why it expires within 30 days and not a year. After 30minutes of checking the system they finally informed me that because I moved from a contract prepaid top up to a prepaid account, it changes the expiry of my data bundle. I am not sure how true this is, and if it is true, why did CellC not notify or advise me of such a change. If I had known, I would not have bought 3 gigs of data, as I only use 500megs a month. Now I am totally of my budget, because what should have lasted me 6 months, now only lasts me a month. I am contemplating changing my service provider to MTN.