Thank you very much for the useful information Adam.
And sorry for a little delay in answer due to connectivity issue.
The most likely cause for this pain appears to be spasm of the abdominal muscles, particularly below left ribs which can occur as a result of unaccustomed exercise, dehydration, inappropriate posture or trauma (which might be un-noticed).
Another important differential is that it can be a radicular pain. Which means a pinched nerve in the spine can be responsible for this pain which happens when there is excessive abnormal pressure over the nerve as it exits the spine (eg in disc prolapse or vertebral degeneration).
Then there are some rare causes such as regional colitis or splenic enlargement which can give similar kind of pain.
Since this has been present for a long duration now, i will suggest you to have this examined clinically by your doctor discussing all the details i have mentioned in order to chalk out the management plan that suits best to your needs.
An MRI of the spine and Ultrasound of the abdomen will be the most useful tests to diagnose this problem with certainty.
Gentle massage of the abdomen.
Pain killers like tylenol or motrin.
All will be helpful.
Just watchfully monitor your symptoms for another few days, and if this persist, does not improve or get worse, have this evaluated by your doctor.
I would have been worried too if I were at your place, but let me reassure you that this does not sound serious at all. It is a treatable condition and carries a good prognosis with appropriate management.
It was a pleasure assisting you and thank you very much for visiting us.
My goal is to provide you with Excellent Service - if satisfied with the response, Please do not forget to Rate my service.
If you feel you have gotten anything less, please reply back, I am happy to address follow-up questions.