Phone Storage is getting low

Phone Storage is getting low

I have a sony xperia neo. I usually had atleast 110-135 mb free in my
phone memory. Whatever new apps I install I always move them to sd if it
is possible to do so for that app. I also keep clearing the cache for
every app almost once a week, infact I also clear the app data ocassionaly
when I think the data is unnecessary.
Recently, my phone's internal storage just reduced by itself. All by
itself. I did not install any new app, infact I actually deleted some.
Obviously I did not move any apps back into the phone memory. Yea, some
apps like YouTube, Officesuite, Adobe Reader etc. that come pre-installed
in the phone memory were updated recently bur these updates obviously
weren't big enough to take up all that memory. I even un-installed updates
on apps like fb which are on the phone memory n which I don't use often.
From having over 100Mbs of internal storage, today I have between 35-25
Mbs available. Why?? Where did all that memory disappear?? How can I fix
this?! Please Help.
Note: 1. I have viber, its moved to sd. Lately, the viber photos folder in
the gallery has started having blank 0kb photos in thousands, all by
itself. I have 5k photos in that folder of which 4.9k are blank and
appeared there all by itself. Could this be a reason why the phone memory
got so low, even though viber is in sd? If not, why is this happening
anyway?
I have avg antivirus (free). My phone is scanned automatically every week.
I did not root my phone.