Timeshift probably, I had someone call me last week with a similar issue. Menu -> setup -> Recordings & Timeshift -> Timeshift settings
Make sure enable autorecord is set to no, if you like to be able to rewind live tv set Automatically start timeshift after to 2 seconds, if you're not bothered about it then set it to Disabled. Then check the Timeshift buffer limit, the lower it is the less space it takes up. I have it set to 4 or 6 on my boxes, Timeshift last events limit, set this to 3 or 4, and timeshift checking older files in minutes I have set to 15.
Hopefully this will reduce the number of timeshift files are stored on your usb stick and therefor less space