Data usage depleted before full use- ****

I have been experiencing zero data without full usage for the past 3 months. I have enquirer with MTN a number of times and they tell me I should keep my background data switched off. At that time, it has been over 3 years that I have been using their network and have never experienced this. They load 50gb monthly but my usage is less than 35gb and when I get to the app to track, my data has been depleted. Their night data is not working, they bill me on my daily data. I am so frustrated.