Charged for data used, but still have gigs left

I've always been on a flat rate R199 per month for 10GB day & 10GB night data, when depleted I had to top up with my credit card. 3 Months ago I've done a sim swop as my device was ******, and then received a bill which had an additional R500.00 usage on it. I went to a Telkom store where they told me my account will be in credit as they can see I still have data left. Then on 12/11/15 Telkom debited R1000.00 off my bank account without informing me. I went to Telkom shop again where they phoned Telkom, as there are 3x additional R500.00 usage billed by now. The lady rep**** that data was used via MTN roaming. I have a minimum of 5 gigs data left each month, so how can I pay for data when I still have left. I never authorised a credit limit, it was always top up. And how can Telkom just put me on MTN internet that I have to pay extra for, when I'm paying Telkom for my inclusive data that I never used up? The Nov'15 bill is R1600.00, which I hope will not be debited, as I've already paid R1600 instead of R400