Unexplained and implausible data billing

I moved from a R389 3GB a month soft cap data contract to a 9GB (4/5) a month last year with a cost of R459.Initially it seemed like a good choice as I do most of my surfing after hours - during work hours I use my office wifi.I noticed some extra charges every now and then but nothing significant . However last month I received a data bill of over R4000 and was told these were out of bundle charges .I found this absurd as my usage hasn't changed in anyway, I don't download media or share my data - my main activity is Facebook and reading the news and all my usage is primarily between 5 and 8 pm per day when I'm home from work. I complained and was told that it was odd and that an investigation would be carried out and I would be notified in 10 working days. In the meantime I paid the said bill while waiting for feedback. Nothing happened. Subsequently I received an email yesterday saying my current unbilled data calls for this month were at over R6000 - less than 2 weeks since the last bill and again with no change in usage patterns! I have reported to MTN who say they will investigate and that there is a credit pending from the last investigation but still no answers forthcoming.