df shown size much smaller than partition size, /home full

I'm having trouble at the moment because my /home partition appears to be full and its usable space seems to be much smaller than the actual disk, which is the confusing part.

this is the line in fstab where my home partition gets mounted:

#home
UUID=6e6a584b-fa83-439b-bca2-1772b65a3cf2  /home          ext4    defaults      0       0

In gnome disks, this is shown as a 256gb partition covering the entire disk (and that's apparently 99% full, which shouldn't be true): link

The confusing part is the following output of df -Th | sort, where /home is shown as only 28gb in size:

df -Th | sort
/dev/loop10    squashfs   88M   88M     0 100% /snap/core/5328
/dev/loop16    squashfs  5,0M  5,0M     0 100% /snap/canonical-livepatch/42
/dev/sda1      ext4       28G   25G  1,1G  97% /home
/dev/sdb2      ext4      117G   28G   83G  26% /
Filesystem     Type      Size  Used Avail Use% Mounted on
tmpfs          tmpfs     3,9G     0  3,9G   0% /sys/fs/cgroup
tmpfs          tmpfs     3,9G   41M  3,9G   2% /dev/shm
tmpfs          tmpfs     5,0M  4,0K  5,0M   1% /run/lock
tmpfs          tmpfs     789M   16K  789M   1% /run/user/120
tmpfs          tmpfs     789M  1,9M  787M   1% /run
tmpfs          tmpfs     789M   36K  789M   1% /run/user/1000
udev           devtmpfs  3,9G     0  3,9G   0% /dev

I just don't get what's going on here... Can someone help me out? My machine is also slowing down significantly in certain situations now where it hasn't before, which I hope to fix after I understand where all the disk space went.

here's another image of the gnome disk usage analyzer, where home is also 28GB: link edit: BTW, .cache/dconf is less than a kb in size

output of lsblk:

NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda      8:0    0 238,5G  0 disk 
└─sda1   8:1    0 238,5G  0 part /home
sdb      8:16   0 119,2G  0 disk 
├─sdb1   8:17   0   512M  0 part /boot/efi
└─sdb2   8:18   0 118,8G  0 part /
sr0     11:0    1  1024M  0 rom