I would say that your available (which should have a high % of the
buff/cache) is very low. Here are my stats running KDE (with many
workspaces and qubes running) and near 6days uptime:
[user@dom0 ~]$ free -h
total used free shared buff/cache available
Mem: 3.6G 1.5G 192M 58M 1.9G 1.5G
Swap: 7.3G 725M 6.6G
I have less buff/cache than you but more available. I have no idea about
what could be happening.