Data Depletion

I am a Cell C prepaid customer, lately my data usage is higher than normal and what I don't understand is that I only have one application running, which is my Whatsapp, In the last 5 days I've spent approximately R100 on airtime. Yesterday I bought R30 airtime and within about a minute it showed 15 megs left. I've had no joy from the call centre as I spoke to two different consultants on different occasions. I was advised by the one that it was my internet usage, but I did not use the internet nor did I download anything. I was then advised be the other that it is my whatsapp but how can that be? I would appreciate your assistance in resolving this issue as I am at a loss both financially and I am quite frustrated by now. Kindly assist in please. Thank you.