Hi Lisa,
You have waited long enough. YES, see a doctor, perferably a Gastroenterologist. The sooner the better. Your symptoms suggest there is something wrong. It could be one of a multitude of diseases which include but are not limited to Irritable Bowel Syndrome, Colitis, Stress, and YES even Cancer. You are too young to play around with your health so go pick up the phone and make yourself an appointment. I hope you will find out that your problem can be treated with a change in life style or at worst a little medication. Whenever you have a change in bowel movements that occured for no apparent reason and it lasts a week or longer, seek medical advice. Good luck.
Paul