Where is my data going???? I am connected to wifi for most of the time!

<p>Last month i got a bill of nearly 4000! My contract is an unlimited contract with 10gb data. I received no sms that my data was depleted, and it just kept on billing me. </p> <p>Again this month my data got depleted on the 20th of this month. I then switched to Wifi. On sunday morning (25th September 2016) I bought a data bundle of 1.5GB. However, I have been connected to wifi for the last 48 hours at home and at the office. I received an SMS today that my data is under 196MB. How is this possible and where is my data going?? My Wifi assist setting is set as off. </p> <p>Again i ask where is my data going???? I have been a loyal customer for more than 12 years, but vodacom has found a way to take my money unneccessarily. I am moving over to a different network (together with my other numbers) after this contract period. Bye Bye vodacom. It was nice knowing you. </p>