Airtime dissappearing in minutes.

I would like to know how Cell C took R64 of my airtime in a matter of minutes. I just plugged my modem in to check my balance & buy data when all of a sudden my airtime balance was 0. I had not even gone onto the internet yet. This has happened before. They can't tell me which sites took the data even though I never had a chance to go on any sites & my browsing history shows i was not on any sites during the time frame of this incident. Also I never received any sms advising me that my data bundle has depleted etc which I normally receive. I buy a lot of airtime & data & if this is how Cell C works then I will rather switch to another service provider.. They quick to take your money but when u have a query you have to wait days or bank on losing your money.This is unacceptable.