This can happen if journals are not limited to a max size. Vacuum the storage
sudo journalctl --vacuum-size=50M
You can limit the storage size by editing the file /etc/systemd/journald.conf and set the maximum log size
SystemMaxUse=50M
It can also happen your pacman cache is filling up. For the pacman cache you can clean it up using e.g.
sudo paccache -ruk0
Or you can use pacman to clean it
sudo pacman -Scc
SYNC OPTIONS (APPLY TO -S)
-c, --clean
Remove packages that are no longer installed from the cache as well
as currently unused sync databases to free up disk space. When
pacman downloads packages, it saves them in a cache directory. In
addition, databases are saved for every sync DB you download from
and are not deleted even if they are removed from the configuration
file pacman.conf(5). Use one --clean switch to only remove packages
that are no longer installed; use two to remove all files from the
cache. In both cases, you will have a yes or no option to remove
packages and/or unused downloaded databases.
If you use a network shared cache, see the CleanMethod option in
pacman.conf(5).
I can also be something in your home folder filling up - most likely your Downloads folder