I think the most likely cause of your pain would be a low back strain, which can actually take a few weeks to resolve. Usually some ice, motrin, and rest will help it resolve.
A few other possible causes could be a kidney stone in your left kidney, but this pain should be higher than your waistline. If this is the cause, you will get increased pain over the next 48 hors and the inability to find a comfortable position. If this happens, you should see your doctor for an ultrasound.
Finally, the loose bowels might indicate that you have diverticulitis, which is inflammation of a part of your large bowel. This presents like a "left sided appendicitis", and often comes with a fever and abdominal pain. If the pain starts traveling to your abdomen on the left side, this is the likely diagnosis. A CT scan and antibiotics would be indicated then.
For the meantime, I would assume its a lumbar strain and try ice, rest, and motrin. Avoid too much motrin by alternating with tylenol (the two are fine to take together). If the pain changes as mentioned above, see your physician.
I hope you feel better.
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