seratonin levels

I have left side pain, I have read that low seratonine levels can cause a person to have pain . I used to take 20 mg of Fluoxetine daily to combat low seratonin levels. I stopped taking it just before Christmas 2007 because it made me gain 60lb of weight, which I could not loose. The left side pain started just around the time when my body would have been free of Fluoxtine. Does anyone know if this could be connected with the left side abdominal pain I am experiencing? I am a slightly nervous person that easily gets stressed out, and I have had  a huge amount of stress to deal with in the past 5 years, about the amount of time I had been taking Fluoxetine for what the Doctor said at that time was Chronic Fatigue Syndrome. I am tired of the pain which comes and goes but always seems to be just under the surface. When I talk or think about the pain, it seems to come on strong. so long as I am walking or standing up and busy the pain subsides. what is going on? I REALLY need help here, I'm not used to being sick and this is dragging me down.