My data and airtime keeps disappearing

I have been using my Samsung S4 for the past two years and have never had issues with data and airtime disappearing. Maybe it's because I was on contract. Since my contract expired and I did not upgrade it seems like MTN is trying to push me into a corner that forces me back to contract. My data has been flying like nothing on this earth! I have restricted my apps to only update when connected to Wifi. Last Friday I recharged with R30 and only watched a 2 minute video on youtube and the following day my data was gone. Last night I bought 100mb and I was on whatsapp for less than an hour. I switched my data off before going to bed and when I woke up this morning my data was gone. Also after buying data I had about R6 airtime left and this morning when I woke up it was down to R2. I called the call centre and some guy kept telling me to check my apps!!! I have changed nothing on my phone since I went from contract to prepaid yet my data keeps vanishing. It seems to me like MTN is trying to force me back into contract and I refuse!!! I've been on MTN ever since I got my first phone and have never had any issues before. But this right here is not working for me!!!