Pain, diahrea and bloating for 2+ months?

I'm a 35 year old woman who has had diahrea, bloating and abdominal pain for the past 2+months.  I've had blood tests and stool testing done and both have come back normal.  I was put on meds for Irritable Bowel Syndrome and that have done nothing.  I was tested for Celiac's and that came back normal also.  I have Lupus but my rheumatologist doesn't think that is the problem.  I am scheduled for a colonoscopy next week but am curious what could be causing this.  Any suggestions?  The diahrea is  daily without fail.