First of all, cardiac issues could be a problem, simply based on the left side under the ribs. It could be potentially a situation where your heart is not getting enough oxygen due to a blockage and that could give you pain. if the blockage becomes complete, and the entire flow is blocked that is when a heart attack occurs.
2. It is not appendicitis. That is located in the right lower quad of your abdomen.
3. gall bladder issues are usually on the right side under your last rib. The pain there can radiate around your right side to your back in a blet like fashion. it can cause pain under your right scapula. based on your symptoms and location I dont not feel its gall bladder.
4. There could be an inflammation of the ribs, which we do call costochondritis. It could be caused by having done something strenuous, like lifting etc. It could be also caused by a bruise
to the ribs and you could have the same sensation.
5. Low on the list but needing to be mentioned is a clot in the lung
. there would be pain, shortness of breath, perhaps some coughing up blood and rapid breathing, as a rule. That needs a special xray.
6. If its lower at the bottom of the left rib cage, it could be a stomach problem or a problem with a swollen spleen. A doctor could do a physical to determine if its the spleen
7.If its radiating or is located in the center of your abdomen it could be your pancreas. You would be fairly sick, with perhaps fever, nausea vomiting.
Thats a start.