Data disappearing

This is the second time this happened to me, the first time I complained they just told me \there might be an app that is open on your phone causing you data to dry up within minutes". This answer left me powerless and still upset because I use my phone every day. I know how the settings and apps look on my phone and I know how long my data lasts usually.<br> <br> So today at 8:04 100MB of data was loaded to my phone (100MB usually lasts me up to 4 days!!). I am a teacher so I left my phone on my desk"