Excessive Bill received after Iphone 6 S upgrade

I lost my Iphone 5S and upgraded on the 20.01.2016 quotation reference ***5 and the handset I got is an I phone 6S. I then found my old Iphone 5S and tried to check my email on my old phone and using my new phone as a Hotspot.<br> <br> On the 28.01.2016 I received an SMS Alert : Unusually high usage has been detected on your cellular number exceeding R6991.04.<br> <br> When I logged on to the Vodacom app I was stressed out my bill so far is R8448.09. I<br> <br> Please help how is it possible to use so much data, I am a loyal customer and my bill has never been so high. I cannot afford to pay this amount which will be debited on 29.02.2016 from my bank account<br> <br> PLEASE HELP ME, have mercy