depending on what you were doing thats a good thing, unused ram is bad
if it was just idle on desktop a possibility is a memory leaked started to happen in a running app, and thus its ram usage would spiral out of proportions
you can check this by going to the task manager and see what application is using what/how much memory
and also you can terminate the offending app when you spot it,
ofc depending on what app it might be, it might be supposed to have a somewhat higher memory usage than others, web browsers, flashplayer plugin, svchost are all some that use alot of memory compared to other apps by default
win 7 can "spike" in memory usage if its either searching for, or downloading/installing an update in the background, this will happen if you have teh corresponding "Windows Updater" settings out on default or any of the auto settings
again high memory usage isnt as such a problem, since the system should use the memory you have when it needs it, thus unused ram is not optimal(you paying for something thats never used),
and free the RAM up for use automatically when another app needs it
it only becomes a problem if the system doesnt free up the used RAM, when you open up a new program, so the program cant use the RAM it needs/wants, and thus the program becomes laggy or freeze
open up a program/game or something semi demanding, and see if the RAM usage changes and you can run teh app without issue, or if it hangs/ performs slow because the ram isnt being made available
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