Take a screen shot (or just write down) the size of the apps in the storage details (General->iPhone Storage, list at the bottom)… Wait a while, take another. Compare.
That should tell you exactly what app is using up the storage. Delete it or tell it to stop downloading/creating/whatever.
Whether you’re using the phone or not doesn’t matter; it’s never really idle, and almost every app that does significant downloads these days does them in the background.
When my gf went through something similar, I moved all of her pictures/video to the cloud. I then increased her cloud storage to the 99 cents a month level. She’s been fine since.
Having just a few hundred megabytes free is really not enough, just one app update let alone any sort of iOS update (which are usually at least 1GB) can quickly eat that up in temporary files alone. You should be shooting for a minimum of 10% free and preferably 20%, especially if you’re only in 16GB territory.
Have you backed up and restored your phone? I hate trusting someone’s “cloud”, so I just backed my iPhone up to iTunes on my computer, deleted all non-essential apps and photos, then restored it (“Nuke and pave”, as an IT friend of mine calls it).
Bingo! Half a phone worth of storage “restored” to me!
[note: the buttons in iTunes say “Backup Now” & “Restore From Backup”]
Another item that often gets overlooked on iPhone storage is texts (SMS and MMS). Remember, those MMS messages can be quite large, especially if you have months and/or years of them on your phone.
Especially if people have sent you attachments, like photos. I figured out how to go into each conversation and delete any photos I didn’t want to keep. With some people (Larry…) I trashed dozens of silly pics that I didn’t realize were taking up space on my phone. I like the fact that I could keep the texts, but delete the pics.
And I discovered that, having finally upgraded from OS 7, there’s now a “Deleted Photos” folder on my phone that keeps those suckers for 30 days (in case you change your mind, I assume). So you have to manually dump those to free up space now.
Some iPhone apps, especially third party ones, are extremely poor at memory management and in releasing memory they’re no longer using. Restoring and reloading the phone is one, albeit time consuming and somewhat frustrating way to clean out that mismanaged memory.
A couple months ago I got a “storage full” message, checked the storage by app and found a massive amount of storage being used by Messages. For whatever reason I had it set to “never delete” texts & attachments (over 4 years worth). I changed that that to keep for 30 days and gained 4GB of storage instantly.