How recently did you update as it might be running background updates for windows or apps
You can go into task manager/performance tab and down bottom click "Open Performance monitor"
open the memory tab (this isn't just ram, its also page file) and sort by commit, see what services are using most private bytes or working set
Private bytes = actual ram usage
Working set = Ram + page file usage
if it doesn't show enough detail...
Download
Process explorer and run it as admin (it comes from Microsoft so its safe)
the default view is tree structure meaning like your task manager screen, it will show what processes are under local host, but unlike the other one, it shows the ram usage of each part so you can see what is eating your ram
This page shows what all the colours and headings mean, link at bottom of it shows how to use it to find problems. You can right click headers and run an av scan from within the program.