*****ing data !!

I had numerous complaints the past few months about data missing on my phone or my tablet. Since I moved over to top-up contract, my data constantly "dissapears...!!!" Yesterday around 13h00 i came home and logged into my MTN wifi. on my phone. I checked my data available on my phone it was 96meg. I never left home. My phone was in the house close to the wifi all the time!! Later last night I checked it, it was 94.17 meg. I left my phone next to the wifi connection.. this morning at around 08h00 i checked it was 91.xx meg. NEVER took my phone away from the wifi.. It is now 10h15. the data is on 89.97 meg. still connected to my wifi!!!! please explain how this happens?????? i will not leave it here as this is *****ing!!!! im not leaving home today. and will constantly check the data on my phone!! whe I was still on contract. I had 400 meg pm. I checked my emails.. my facebook, pinterest, and sometimes google as well. I NEVER, EVER used the 400meg data in a month!!! suddenly when I changed contract my data is sometimes more than 1 gig pm!!! i deleted all apps. hardly ever check FB and my emails is not on my phone available anymore!!! i need an explanation on how it is possible that the data dissapears!!