Over Charging

I have a data contract with MTN called use your own where I receive 1GB a month. It costs me R69 a month, and when my debit order went off this month, they deducted almost R2000.00 extra off my account. When going to the branch where I took the contract out, they explained to me that I used more data this month which is not possible as we live on a far and are unable to connect to the internet. We only use it when in town which has not been for more than a month.She than explained that they would have to request my paper work from the head office and it can take up to 30 days to try and get a credit passed, and unfortunately MTN is striking so it could even take longer. I was unable to pay my rent this month due to this which has caused me to go into debt which I feel is not right. When trying to phone someone to find things out there is just no response. I am not happy at all, and wonder if MTN is the provider they say they are.