Assuming you haven't taken a nasty blow to your left side, I would say it's unlikely it's an organ (spleen). Pain isn't consistent with a bruised spleen either. Kidney pain is usually felt in the back just above the lumbar region on each side. Often it radiates downward to the groin on one side or the other, too.
Pretty much sounds like muscular to me. Particular if there aren't any GI symptoms (loss of appetite, nausea, vomiting, diarrhea, changes in stool, etc).
Sometimes it's difficult to nail down exactly WHICH muscle was strained, but honestly the treatment for strains doesn't depend on the muscle. Rest. Motrin if your stomach can tolerate it.
Thing about Motrin that a lot of people don't realize. It has two properties we love. First, it's a mild analgesic and relieves some of the pain, usually with the first dose. Second, it's a great anti-inflammatory that reduces swelling. Unfortunately, in order for it to get to the anti-inflammatory stage, you need to be on regular doses for a few days (in the military we used 800mg three times a day, but many people can't tolerate it, particularly if you have any stomach issues like ulcers or anything like that).
I always told my patients that if you take a few doses and the pain goes away, keep taking it because all you've done is relieved the pain, you haven't done anything to relieve what CAUSED the pain (inflammation from an injury). If they can stay on the Motrin for at least 3-4 days, three times a day (doesn't have to be 800mg either...200-400mg sometimes will do the trick) the level of ibuprofen builds up to a therapeutic level for reducing inflammation and fixing the problem. After that most people can drop down to 1-2 times a day to maintain the level. I'd tell my patients the pain should feel better after the first day, but if they stop taking the Motrin it will probably come back. After a week of rest and Motrin most patients are ready for stretching exercises and mild activity, building up to full activity as tolerated. Any pain means back off. There's no such thing as "no pain no gain" when it comes to rehab.