Mac storage usage

I have a mid 2014 macbook pro, with just 128gb of storage. So as you can imagine, space is at a premium. When i've left it on for a day or so I notice my remaining space goes down. Is this some sort of caching or system restore or something that's eating up all my usable remaining space? I have about 5gb free, but the end of the day it's normally full. Is this normal? Or is something leaking

All I do to clear it is reboot. The following screenshot is after and before a reboot

When my Mac was almost full it used to do that. Usually 2 or 3 GB, i believe that is really a kind of cache. I upgraded my storage with a SD adapter and it stopped to happen.

Mine is from TheMiniDrive, now i have more 256GB hehehe