Data Vanished

My data keeps on disappearing from my phone. I went from 2 GB and R300 balance 2 days ago and now I'm at zero for both when I'm hardly on my phone during the day. At home I'm strictly on wifi. This is not even the first time that this is happening where I lose an enormous amount of data in a short space of time. I'm not sure if there's a glitch with the tariff system or if Vodacom is deliberately overcharging, but this is mind boggling and ridiculous. At this point it seems a futile exercise to even recharge my account as my airtime/data will just disappear out of the blue again. Very disappointing....