I am a 19 y/o and have had diarrhea for a week now. and have had constipation prior to this and the left side of my colon is sore.?

You can say: you have diarrhea if you have at least three loose or liquid bowel movements each day.The MOST common cause is an infection, most often a virus--viral gastroenteritis. You can usually just watch and wait, but after a week, tests should be considered including stool cultures. The fact that preceding this you had constipation raises the question of irritable bowel syndrome. See your doctor for tests