What causes constipation that lasts for a week?

Many causes: Lack of fluid and fiber are usual causes of constipation, others being laxative abuse, medication effects, neurologic disorders, inestinal obstruction etc. See this site for more info. http://www.mayoclinic.com/health/constipation/ds00063.

Several variables,: the most important being internal control of colon motility, delayed evacuation (for whatever reasons), internal emotions, decreased physical activity, etc. Most stimulant laxatives ↑ constipation problems over time. Generally ↑ed activity, ↑ed hydration, high fiber foods, prunes, softeners, osmotic agents, in order listed, are best/safest ways to ↓ & prevent constipation problems.