5Gb data disappear over night!!!

<p>I've recently upgraded from the S4 to the A5. I had 7Gb of data left. As per my phones data usage I've only used around 2Gb and have updated all my apps over wifi with auto update set to wifi only. Last night when I checked my data was still around 5Gb but when I checked this morning there was less than 400Mb. How is this possible?!! I'm posting here instead of callinh them because I'm sick and tired spending my airtime to try and reach them, which takes forever only then to end up in the wrong place or talking to someone who can't help. Going into their shops is not much better and sometimes even worse. This is outrageous and unacceptable!!! I need answers and solutions.</p>