Just try with following steps....

Tap Settings > General > Storage & iCloud Usage
Tap Manage Storage.
Tap an app taking up a large amount of storage.
Take a look at Documents & Data - if there's over 500MB of documents in the app, it may be worth deleting it and reinstalling it to clear the extra space.
Simply tap Delete and head to the App Store to re-download it.

So two weeks ago my iPhone 6 kept filling up with storage no matter how many apps or pictures I deleted. It eventually lead to my phone being unusable besides for texting and calling, and then it went into a continuous loop of rebooting itself and I couldn't do anything about it. But next day it started and worked well. Now today, it's starting to do the same thing. I currently have 41 pictures on my phone but it says photos take up 2.1GB of storage. I'm no math major, but that **** doesn't add up. Has this happened to anyone, and if so, is there a cure? I am in Hawaii and thinking about taking it to tech store such as TCAWireless.com or any other. What should I do? Please guide.