Storage space running out, where can I find more?

[AC Question]

Storage space running out

I have a droid Maxx that constantly tells me my storage space is running out but when I open the storage in my settings, and leave the page open for a while, suddenly 5+GB of space is free again. I know that there is plenty of space on my phone because I don't take pictures and I haven't downloaded any new aps in months...