Your data breakdown should give you a better indication of where your data went, have you checked that? What does it say?
Prepaid Mobile or Prepaid Mobile Broadband?
Did you happen to donate data?
What phone do you use? Have you checked the data usage in the phone settings? It's also recommended to align your data usage calculator in your phone to your monthly billing cycle.
Daily usage in your data breakdown or as per what your phone itself says?
Can you share a screenshot of your phone's data usage calculator?
When you hit "Reply" you should see a window between the body of where you enter your text and the "Post" button which says "Browse files to attach (maximum size 5MB)"