Data depletion

I recently upgraded my Cell and iPad package to suit my pocket better as well as see the best I could get for my buck. But am finding that recently I received my data upload come through on the 17th of each month and unsure why? I have been to a MTN merchant and asked but they keep on telling me that is my contract and will send me a copy of it. But the reason for my grip is that I receive my data on the 17th and by the 26th it is finished. I don't go on to my phone a lot and utilise the WiFi at work during the day so just cannot understand how I can go through so much data. My iPad I hardly ever use and see that has also depleted badly....why? Feel I'm been taken here and thinking of cancelling my contract. Why does the data deplete so rapidly? Please explain?