I suggest first checking for malware. After making sure the system is clear, you need to check Windows Task Manager by clicking ctrl+shift+esc and see the processes and their memory usage. Some tasks might not show up, so click on features like "show all processes" or equivalent to show them all.
Next steps come after that if this does not show the culprit.
Click on the Processes tab then arrange the Memory column to descending by clicking on it as needed. If there is a "show processes from all users" option in the same tab, click on it too.
This will show what is eating up the memory, if it is something not hidden like a malware or something.