I'm sorry to read of the discomfort you are in, and hope I can give you some advice regarding it. Of course, online no one can state with any certainty what anything really is, all we can do is give the best guess based on what it written.
Pain that is reproducable, meaning it comes again when the area is touched, or on a deep breath, most often is of muscular origin. That is why I asked if you were coughing, or did heavy lifting. Those types of activities can pull the muscles around the lungs; a term that is called costochondroitis. Normally, I would expect to hear about a recent cold, or other activity that could explain this, but just because you haven't had one doesn't mean it is not the pain of costochondroitis you feel.
And of course that doesn't mean it IS it, either. The "big bad wolf" of left sided chest pain IS of cardiac origin, the pain of a heart attack. The pain of a heart attack CAN be felt in the shoulder and neck, as your pain is. Most pain from this is NOT reproducable, however. But, I cant say the pain you feel is NOT cardiac. It is, however, one thing that CANT be ignored no matter how minor a chance it might be; online its always wisest and safest to err on the side of extreme caution. And you do have enough signs to make me consider it.
Its possible also that you did have a bout of indigestion or gas; trapped gas under the diaphragm can be exquisitely painful, but it normally does go away without leaving any residual, and you still have some pain.
You were right in that left sided back pain can be from the gallbladder (and liver); however, pain there would increase after meals, especially fatty ones. Cheese is high in fat; your pain didn't worsen. A gallstone, though, might cause this type of pain that you describe, and if it shifted, that may be why you feel relief. Of course, I cant write that its not gallbladder related, either.
What should you do? Given the slightest chance this COULD be cardiac, I would have it checked out. A pain level now of 3 or 4 isn't major pain, but I have no idea what happened prior, and if the pain then was so bad you were considering going to the ER, it must have been severe
You don't quite fit the textbook case of costochondroitis (no known precipitating factor). IF this was costochondroitis, it does eventually pass on its own, with self care measures like OTC pain relievers and heating pads. If it continues past a week, then a doctors visit is warranted. But I cant state it IS this. It may indeed be gallstones, and they may have shifted so are no longer blocking ducts.
I am not quite comfortable with the idea of you leaving it alone though as I did consider it may have a cardiac origin. I do recommend you be evaluated, and as there is that risk of it being heart or cardiac related, I do suggest as soon as possible.
Here is information on costochondroitis: http://www.mayoclinic.com/health/costochondritis/DS00626
Here is information on heart attacks: http://www.nhlbi.nih.gov/health/dci/Diseases/HeartAttack/HeartAttack_WhatIs.html
I hope I was able to help,