Vodacom data *******

Ive been having this contract with vodacom for over 5 years now and 3 years back the same thing happened where data started disappearing after i decided not to upgrade the contract and now it happens again i haven't yet decided to upgrade the contract which is past due for upgrade so data has been disappearing i get 30GB of data on the first and it last me until the 25th max 28th every month however i find it troubling that on Wednesday 10th May i checked my balance and it was less then 15GB which was already odd besides this is my personal pocket router and no one knows the password, so Thursday 11th May while at work i received an sms on my router which i checked after getting home the it stated that i have less then 8GB left i was like what the F*** before i went to bed suddenly i have 5GB of data left so can vodacom explain to me the actual hell thats going on coz now im paying for magic tricks.