Is the colour of stool dark green in colour?
have you noticed blood in stool ? Fever?
Vommiting? Are you accepting fluids?
Since how long you have these symptoms?
Dark colour of stool occurs due to decreased colon transit time in passing loose stools.Diarrhea decreases colon transit time, and thus dark stools. As such it seems that you have viral originated diarrhea. Maintain your fluids levels, Dont take any solid food , You can take Tab Imodium to reduce the frequency of bowel movements . it can be taken maximum up to 4 times , as per the requirement .
It is most likely of viral origin.You should wait and watch, Slowly symptoms will get resolved. If you notice fever, Blood in stool, any sign of dehydration then consult your doctor.