Low storage notification with 18GB Free!
Sounds like a corrupted file in the operating system. 

What i would suggest doing is reformatting back to factory settings.

on the phone go to  Apps > Settings > General 

Tap Backup &amp; Reset uncheck Back up my Data (if you want a clean slate). Leave it checked if you want to keep your pitures and other documents.

Tap on Factory Data Reset after doing so it should start reformatting

PS. also remember to clear your sdcard cause there might be of a virus
i would also get a antivirus for your phone as well. 

Hope this helped :)