Mobile Data disappearing fast whilst connected to WiFi !!

<p>Have been a Vodacom pay as you go customer since January and what I am about to describe has never happened before. I monitor usage carefully and have checked all settings on my phone (iPhone 6) to ensure that mobile data is not used unless absolutely necessary - and certainly not whilst wi fi is on. Most of the time I am connected to a wi fi network, at home or at work - and always connect to wi fi when possible in a public place. Much of the time I turn off Mobile Data to avoid "accidental" usage. So I use Mobile Data mostly for checking emails, news and weather, when on the move - only if I have to. I last had 400mb (according to Vodacom) on the week end. Last night (Tuesday), after very little mobile data usage through the preceding 3 days, I was streaming some TV from my iPhone to my Apple TV using home wi fi. The signal was strong (I was in the same room as our wi fi router) and everything was working fine. Suddenly, I started receiving SMSs from Vodacom telling me my available data was being used very quickly. I immediately went back two screens to check the wi fi signal again - it was still on and strong. So how is it possible that I chew through over 100mb of mobile data whilst connected to wi fi ? I have read other users' stories of disappearing data - I think it just happened to me ! I would like Vodacom to reinstate my data please. </p>