Disappearing Airtime

For the past few weeks my airtime has been disappearing. Cellc gives you free data bundles when you recharge, but every time I buy airtime or a data bundle my airtime is the first to be depleted. Just bought a 250MB Nite daily bundle and now all my airtime is gone! I've called the call centre before and they have confirmed that I have NO subscriptions. I also disabled auto updates on my phone. This is really frustrating. I need to know why my airtime is always being used up when I still have data?