Depletion of data bundles without valid reason.

I purchased a 100MB data bundle on Saturday afternoon. After an hour, I received a text saying that I had 4.36MB remaining of my data bundle, having only used my bundles for whatsapp. On Monday morning, I bought another 100MB. I experienced the very same debacle. I called the service provider and they sent me instructions on how to turn off background applications, data roaming etc. I followed the instructions and loaded another 100MB bundle. After less than 30 minutes, I receive another text saying that i have 4.59MB remaining. A few seconds later, it has depleted. What do I do? I'm a student and have spent over R100 on bundles that I have not used. I'm extremely angry.