It is possible to accidentally make off-canvas content and in that case, as @dandelier mentioned, Image → Trim to Image Size will get rid of it for you.
However, it does seem that more and more RAM is used as you do more and more operations.
You could try reducing the size of the Undo levels at Settings → Configure Krita → General → Miscellaneous tab → Undo Stack SIze in an effort to reduce the effect.
Otherwise, doing Save then Quit then restart and reopen is the way to deal with it.
This has been mentioned before, here:
RAM vs status bar
If you’re setting 8GB as the RAM limit for krita and you only have 8GB of RAM on your computer (you don’t say how much RAM your PC has) then this will cause a problem as OS level swapping starts to happen as well as krita’s own swapping activity and things do get very slow. (On Windows, crashes happen.)
I suggest that you set the RAM limit to 2GB less than your total RAM and don’t run any other applications at the same time.