Experiencing upper left abdominal pain just below the rib cage, more like discomfort and fullness in that area, can be due to swollen and enlarged spleen.
This pain can sometimes spread even to the left shoulder.
You may experience an early satiety, since the swollen spleen presses on the stomach.
The enlarged spleen is often tender on touch and if you noticed sever pain that is getting worse when taking deep breath, you should visit your physician right away.
There are many causes for enlarged spleen, like viral and bacterial infections, liver diseases, anemia, cancers.

See your physician!