Data Line not used but billed

Vodacom blocked my line for no reason. When visiting the store I was advised that the data line linked to my cell line has an outstanding invoice of 8k and this has now blocked my cell line too. The soft block was released on my cell line. My data has had a soft block on it since the onset of the contract. Every month the line becomes unavailable for usage as it reaches limit. How all of a sudden was I able to use a R8000 on data??? Neither the store assistant or the call centre agent I phoned was able go advise me what I used the data on. Best of all is that the ****le that this data line is in was switched early in the month due to me not being able to use it because it was capped. I need clarity on this and proof before Vodacom takes R8000 off my account for something I did not use.