Limit depleted when I didn't even use it

MTN keeps on ripping me off. My bill closes on the 13th of every month and from the 14th I can use my purchase limit which I have set to R1000. I tried to buy data using my limit on the 15th of May 2018 after my 500MB was finished very quickly in just a couple of hours and I was getting a message that said I had insufficient airtime to buy data. I called MTN and they confirmed that my limit has not been used it needed a reset but also there was a mysterious bill of R167. The question was then how did I incur that cost if the limit needed to be reset? A call was logged on the same day and on the 16th I receive an sms that said my limit had been refreshed and my limit was set to 0 and I could set it to whichever amount I wanted of which I then set it back to R1000 and tried to buy data again and I was getting the same message that I had insufficient airtime to purchase data. I then called MTN again and they told me that my R1000 limit has been used up and couldn't tell me how? They said their systems take 24 hours to refresh then another call was logged again. How can I finish 500MB in a couple of hours then deplete R1000 again in a couple of hours when I didn't even buy anything?