For the past couple of days I've had pain under my rib cage and back on my left side. It hurts more when I cough or make certain movements. I don't really have any trouble breathing unless I take a really deep breath. What should I do?
There are a number of possibilities: pleurisy (inflammation of the membrane around the left lung), chest wall irritation causing intercostal muscle pain, diaphragm muscle inflammation, spleen enlargement and pain, intestinal inflammation, kidney inflammation or stone, even stomach or pancreas problems.
Most likely it is muscular since you don't feel sick or report other symptoms, but there is no way to be sure without an examination, and even then, it may not be possible to determine the source of the pain. If over the counter tylenol or ibuprofen is not helping, then it is time to be examined.