Look at how you are eating. Do you consume a lot of caffiene? If so, this can really affect your colon. It can cause things to move along faster than they normally would, causing frequent defecation.
Other things might be too much chocolate; medications; water; pain killers such as aspirin, or even milk because some people are sensitive to it.
I would first look at caffiene consumption, keeping in mind that each person is an individual and what causes this issue in one person may not in another. Consuming a lot of caffiene long with aspirin would almost certainly cause issues.
Additionally, get checked by your physician for things such as colitis - but first cut down on the caffiene and other things as noted. Also, if the stool appears dark, despite these changes, be sure to see your doctor, and don't be embarrassed, like my daughter was. This has to do with your health and is pertaining to normal body functions.
Take time for fun!