Confused and Frustrated

I have a cellphone contract with MTN but have constantly bought additional data through my FNB app. My general monthly bill is between R800 - R1200 per month. In November 2014, April 2015 and May 2015 my monthly bill was above R3000. In each of those months when I had checked my data usage through the 141 service I was generally always in excess data. <br> November 2014 query: I was told that the team would investigate and they would get back to me in 7 days, I got no reply but for the two months post that I had been credited on my invoices which clearly showed that there was a mistake.<br> April 2015 query: I was told that the data I was buying off the FNB App was only night-use data and therefore while I had available data for night usage my airtime was being converted for day data usage. I told the customer rep that this was not clear on the 141 service as there is no breakdown between night and day data, so as I consumer I am blind to this. This is under investigation by FNB who I have contacted. <br> May 2015 query: I have requested both the April and May bills go through the same investigation. This process takes 7 days, during those days my bank account will be debited with over R5000.