Account data billing

Bought two device's on chat 100, Huawei P8 to be exact. Both on the same contract. One device unreasonably uses data at at abnormal rate that it had used up more than 15gb of data only on social media usage. The same device only used less than 200 megs. When I report the matter to the customer care all I get is that I have been on the net. I explained my self nd also tried to block all applications from accessing data on the settings with the agent and the problem persist. For me to send one message on what's app it deducts 6 megs from my data. I explained this to few customer care agents who keeps on insisting that I am the one goin on line. Can someone please try to fix my problem without having to make judgment as I have called in more than enough to always have to start explaining my self again and the agents to ask me to call again as I did before. At this point in time I can't even access my emails, cannot go to the Internet as am afraid that I will do more damage and cost to my account. Someone please assist