Data theft

I upgraded on 11 May 2019, for which the first payment will only be on 1 August 2019. Therefor my current contract, on which I receive 5GB of data every month, and for which I will still be paying up to then, is still active. What I cannot understand, is why they, all of a sudden, after I upgraded, deduct 4 GB of data from my package. For June and July I will only receive 1GB of data, although I still have to pay my full installment, which include 5 GB of data.

This is *****ing!!!!!! Ive been to the Cell C shop in Rustenburg and called customer care as well, but nobody can give me a reason why 4 GB of data was taken from me, although I still have to pay for it. Do I get punished for upgrading in some way???? Can somebody who is capable please contact me and explain to me how Cell C can do this. Im freakin furious.....