Data storage problems iPhone 5s iOS 9

Hi everyone,

I've been having this really annoying bug for a while now (few months, but omnipresent in the last 3 weeks) where my iPhone simply seems not to know the amount of storage it has. I have an iPhone 5S with 16GB of space running on iOS 9. Keeps saying it's run out of space, but every time I free up some (by deleting photos, apps, etc) it keeps filling right back up. This one moment I had freed up 200mb, and within 20minutes it was back down to 0 without me interfering with it. As a result all my iCloud content has either been deleted or is uneditable from my iPhone (Notes, Calendars, etc) and most of my apps either always crash, don't work, or downright just bug (Messages always showing up as sending when they've actually been sent, Facebook and Messenger keep crashing, my games don't work, I can't take photos, etc). I've assumed that this is the iPhone's natural response to being low on storage space, but thing is I haven't been filling it up..haven't taken pictures in nearly a month, etc. So before I go over and do a hard restore and backup (iCloud isn't backing up anymore->everytime I try it it just says Backup could not be completed), I was wondering if anyone had any ideas as to what might be going on and what I can do to fix this. Been bugging for me for quite some time now, and if it this doesn't work, I'm switching over to Android, with it simpler operating system. So if anyone has any ideas, feel free to share.

Thanks!

Tristan