Shocking charge all of a sudden.

So I've been using Rain network since last year aroud July. My problem started now when I downgarded my data plan to R250 from R479.00 in march znd my bill came up R958 or so. Pleas3 note that I pay every month and when I dont pay they switch off the data. So how can one be in arrears?? Anyways they clain to debit my account on the first but they cant provide me with the proof. I provided them with my credit card statements to show there there was money on the 1st if the month and there was no debit from them... They debit me just after the 15th or so and when ther debit order does not go through they swith it off. Now I've been paying around the 20th almost since i took their sim card.. Now all of a sudden they changed their billing system and now I owe R958?? What does they billing system have to do with me. I've paid every month. They mustn't give me the story about them debiting on the first of every month. I have not bern paying on the first of the moth in all the months I've had thus sim.. Why all of the sudden I lust pay double.