*****ing data

I have 3 contracts with MTN, 2 old ones and a new one. I never had a problem with my data until the beginning of September. My data would last most of the month but suddenly the last 2 months it doesn't even last 2 weeks!! How is possible for data to disappear during the night when I'm asleep and mobile data is switched off? I take screen shots of my available data when I go to bed and when I wake up in the morning sometimes even 20MB is gone. This is ridiculous and I'm considering cancelling my contracts if it does not get sorted!