3 wks chronic diarrhea?

I am a 39yr old female and am healthy, all things considered.As a standard I would have 1-2 bowel movements weekly (even as a baby,per my mom). For the last 3 weeks, I have had consistent diarrhea. Some times mushy with floating particles, and other times just liquid. There is no fowl odor accompaning these bathroom visits more than once daily. I awoke at 3am with the urge to go and I did. It appeared as if I had pooped out a 2 inch long black string. How long should I wait to call my doctor, and what could be going on. Shaking on the POTTY, Becca