I want to know where my data went to

I have a router contract with telkom,within two days my 10gigs of data was depleted and they are telling me they don't know what happened to my data and I asked them how is it possible to use 10 gigs of data in 2days using whatsapp and they telling me they don't know what happened,what's the point of paying for something I didn't even use,I want to know what happened to my data and I'm the only one with a WiFi password.