Data missing

On Friday my partner got a call about upgrading her data contract, from a 20gb to a 50gb including anytime minutes, She did the upgrade that was supposed to take effect 1st May, it actually took effect later in the day on Monday now between friday and today (Wednesday) she has apparently used over 50gb of data and the payable amount on the bill is already over the amount of what the lady said this new contract is going to be. Customer care cannot explain it to us, vodacom shops can not explain it to us. I am getting really tired of this kind of service from vodacom. If this is going to be the case cancel the contract and we will move to another provider.