MTN Billing Highly Deceiving

During the period of 13 Feb and 13 March 2015 Mtn had recharged my 200MB and R200 airtime. I would use the internet and social network like any other month. However, this time around I noticed that my MB are not charging as usual so I would check the MB tally frequently to make sure I do not overuse MB. Therefore, I would check my MB and Airtime frequently to access the activity between the two in regards to my internet browsing and social networking. The MB would be only charging 0.8MB or so daily which I thought MTN was being generous this month. Then moving on to the beginning of March my MB were still quite full, knowing that I had not overspent any MB or Airtime I continued browsing and social networking- doing so I would check my MB still this time I kept a record of MB during the beginning of March. According to the MB tally I had not overused any MB nor airtime during the period mentioned now MTN says I owe them R1500- which is highly frustrating and confusing since I made sure I do not overspend.