Why is the local disk full?

Ever since last Sunday the local disk on the laptop has been full. I repeatedly empty the computer of useless information (as best as I know how, using CCleaner and uninstalling unneeded programs) but it just fills up again at a very peculiar speed.
It’s never been near full before this, and I am quite stumped.
There have been no changes in usage. The only thing different that I can think of is that I hooked up to a public network last Sunday, but TrendMicro isn’t picking up on anything.

Thank you,
Feeling Clueless