Ironically, I had similar pain that started about 8 years ago as well. At the time, I was seeing an inept doctor (not too much later I, my husband and parents all quit seeing this "quack" luckily... without going into detail, the change saved my mother's life!) That doctor was stupid enough to say "there's nothing over there." Well, there is...the spleen, kidney, stomach. My pain, like yours, came and went over the next several years and I kept hoping it was nothing to really worry about, so ignored it. A routine blood test recently showed that I am very aenemic and have lost over a quart of blood internally. It was then that I admitted about my escalating pain in the stomach and to the left side and back. I just went to see a gastroenterologist. He, along with my doctor, sent me for more blood work/tests and urine test (just did that today). He has me scheduled for an in-hospital encoscopy and colonoscopy procedure for the fact that I may need a blood transfusion and due to the fact that I've been on strong pain medicine including INSAIDS for 12 yrs and wouldn't be able to put me out without an anethesiologist. He and my doctor also will be looking for signs of an enlarged spleen, which can cause those dull and sharp pains on the left side and severe aenemia and will check my left kidney along with other kidney and liver. I strongly suggest that you not give up until you find a doctor who will listen to you! Our bodies don't hurt for no reason; they're trying to tell you something. Did they do extensive blood work ups/urine test? What about a sonagram and/or an MRI vs. just the CAT scan? Don't give up or give in; something is wrong; find the doctor who finds out what!
Try looking up something about floating rib or rib pain. I remember something from another website where there was a discussion about rib pain and it being related to floating rib or something similar.