I'm pretty sure it's spleen pain. I started having the exact pain right underneath the left ribcage when I got really sick with mono... I've had it intermittently since but it's been very noticeable the past few months...
accompanying this pain is very noticeable tingling a bit higher on my upper back between my left shoulder blade and spine. These sensations have been just as noticeable if not more so than the liver/gallbladder.
The first few times I did castor pack on this side, I experienced some very noticeable reactions.. and I felt worse after starting to do them.. this has lessened and I feel better overall after a few days of this.
So... I'm not sure :/ but I'm continuing a few hours on this site and a few hours on the liver every day.