Q: disappearing phone memory Gb

I recently got a new Iphone 5S from online that was unlocked, never used and reportedly not jail broken.  I have not yet connected it to the internet but it was reportedly fully updated as far as the IOS goes.  When I was watching a movie that I had managed to transfer to is from iTunes, a message suddenly popped up on my phone saying that the memory on my phone is "critically low" and it recommended that I click on settings<general<usage to manage the memory.  My 54 or 56 Gb phone showed that I had an available balance of memory of 14 mg.  Then all of a sudden when I clicked back to "general" and then back again to usage, it showed a balance of 0 Gb. The day that I transferred the videos onto my phone, it showed a remaining available balance of over 30 Gb.  By the time I handed my phone to my husband sitting next to me, the balance had started creeping up.  Several hours later of periodically checking, it was back to 30 Gb.  Apart from that my phone seems to be functioning normally.  I just don't want to have an oversight of a bigger problem or remain with this phone if it is going to keep giving me problems.  Any suggestions or ideas?  It is now back down to 29.6 GB from this morning but I haven't connected to the internet, taken any new pictures, uploaded anything.  I have only made a couple of phone calls.  What should I do?