Exorbitant billing

Good day<br> <br> I don't understand how MTN has been billing me for the past few months. I remember making a call to MTN and spoke to a lady who agreed to put my limit to R500 for data because I mostly use my work cell phone to make calls. On 6th April 2015 I received a text message warning me that I'm about to exceed my credit limit of R 2700 and on 14 April I received another text message saying that I have exceeded my credit limit of R 2700 and they suspended the line. End of May MTN debited R 3000 from my account without any prior warning and how do they get to that amount if your credit limit is R 2700. I have never in my life paid these amounts EVER. I use my tablet mostly for internet and my other phone to make calls. Its beyond me how they get to these amounts. I used to pay approximately between R 400 - R600. In April when I phoned after holding for houuuurs I was just told I use too much internet and they were not even willing to look at it. I'm due for an upgrade and will not renew my contract with mtn if this is not sorted out. This completely threw me off budget. I don't know how I will get through the month.