What Is The Cause And Treatment For Continuous Diarrhea?

Hi. I have had diarhhrea for a couple of months. i; ve tried various things to stop it including a medicine for parasites because i have had one before and the symptoms were simialr but not as severe. the medicine was metro.... somehing. i haven t lost weight , i feel fine otherwise i eat. . sometimes i feel a little nauseous but nothing bad and no bad stomach ache. i ve told my doctor and we are going to do a colonuscopy and other stuff. I m healthy otherwise and seldom get sick with anything. i think its a stomach problem but what do i know. any ideas what it could be excluding the big C ?