bad habit of taking airtime

Cells C has a bad habit of just chowing my airtime then their excuse is that I use to much internet but that's not true because I usually use the school's internet like now. After calling to find out what happen they don't give a valid reason. I recently revived airtime from my mom and Monday when i checked I had R90.00 but last night it was all gone so are my SMS's, data and free minutes to to cell c to cell c numbers. Apparently it expired after 5 days. They gave me no worming, just nothing and its not the first time nor the second or third, they constantly doing that over and over again. Last night I asked to speak to their manager whose name is Sean Mkhabela. They did nothing really just apologized and said they can't do anything about it but the sad part for me is money wasted. I bought airtime but i can make calls or anything. Please help but I feel as though its a *******.