Data disappearing

My phone is always connected to my wifi at work and home, so I don't need a lot of data. I manage to get through with 1GB per month on my phone. Mine data was finished by the 29th. Decided to load up 200MB that afternoon as on average I use about 250MB per week, and I only needed data for 36 hrs before my new data for month was allocted. In 24 hrs I received a message to say all 200MB been used, which does not make sense as it was a normal day like any other day, and I was asleep for 8 hours and phone was connected to home wifi. I can't understand how suddenly could use up 200MB in one day where it normally takes me a week. Did phone the call centre, they suggested I put off background data etc but I explained that was all on for all the other days, so why will it be different now. She wanted to send me the itimised billing to check but at a cost of+- R22 per page. When I said I am going to contact head office to find out why my data is disappearing, some of my friends have the same problem, she suddenly offered to add data on for me as a gesture of goodwill. Hopefully a private company will start investigate these companies to check for *****.