Contract *******

On the 4th of April, I renewed my 2 year contract with Vodacom. Since Vodacom doesn't have the 135 Top up deal anymore, I signed a contract for 'u choose flexi' which gives me R 200.00 a month airtime and data. I have always had enough data and airtime on the 135 Top up and thought that this new contract would be more than sufficient. On the 22nd April, I checked my available airtime and I had 22c left! This is impossible! I certainly did not make more calls or send more sms's than usual. My mobile data is switched off most of the time, because I use WiFi from the router in my home. Please explain??