My data just disappeared... I only just reacharged. Normally I use my phone much more than I have today.
I have looked at my data usage however it does not show why is has been decreased so dramatically.
Does anyone know why this happens?
Check on the phone itself - you can see which apps are using data.
Unfortunately that only works well if you reset your usage amount on the phone when you do a recharge (something I recomend)
MYACCOUNT here on Optus should have some broad indication of usage categories (might take 24 hours to fill out)
I assume you're not watching any youtube or netflix?
The other big drain on data is apple itself as it might be trying to sync big changes in movies, music, photos, files, backups, etc.? Perhaps check your phone isn't set to do these things over 4G