Data and Vodacom once again

Please explain to me how can my data be finished if my phone is lying on the desk for more than an hour without anything happening on it or it being touched, all apps including whatsapp and safari data usage has been switched off and all of a sudden sms's came rolling in out of nowhere saying "you have less than 50MB of your vodacom data bundle " followed by another one "you have less than 39MB of your vodacom data bundle " and last but not least "Your vodacom data bundle on 082... has run out".....???