Q: I lose GBs without any reason. Please help

hey there

first of all i am no native speaker, so, please don't mind my mistakes.

in anyway, i got the following problem: i own a 16 GB iPhone 5S bought in January 2014. I use it since October 2014 until now.

One problem that came up a few months ago was my limited storage. I only had another 500 MB free (although my apps + music/photos and stuff) did not fill up more then 6 GB of my 11 GB (usable) capacity. When i connected with iTunes (what i usually never do) I found more than 5 GB of "other" data showing up on the screen. 

I saved a back up on my PC, reset the phone and installed the back up. The other data were down to 1.71 GB

I had free 7.1 GB on my iPhone - that was yesterday evening at 10 pm.

Now it's 7.30 in the morning - so 9.5 hours later and i already lost 1 GB. My free capacity is down to 6.2 GB in not only half a day!!! I didnt take any pictures, downloaded any music or did another thing that could possibly explain that.

So, what is the problem? Why does my phone save up this high amount of data I obviously do not need? If it keeps that speed I will be down to 500 MB again in a few days...

Would be glad if you could help me.

regards

yuurei_sui