Happened to me once. Backed up all my data before performing a hard reset. The issue has not returned since then.

Make sure your computer has iTunes and plug your phone to it. The software will automatically perform a complete back up of your data. Once that is done, you can then perform a factory reset.

Alternatively, if your have vast iCloud storage capacity, you can back up your data via iCloud. However, this only backs up your most essential datas i.e., it does not back up all your data.

So you have 2 options to go around.