It's hard not to worry about your health when you're on the toilet 293847 times a day. But, it is a very common occurrence, even for "normal" people, to get a bout of diarrhea that lasts several days every so often. Do you have any other symptoms, like severe abdominal pain, weight loss, fever, etc.? This can often give you a clue as to the severity of the episode. How long does your D usually last?
And, welcome to the forum!
Its not really necessary for family members to have cancer for you to be at risk. They found polyps in my mother when they did her first colonoscopy in her 30's. I am tested every 2-3 years that started in my mid 20's because of family history of polyps which most doctors think are just as important. My last colonosocpy at 29, they found and removed 3 polyps so its best that you really understand your family history. I mean, what average person would suspect polyps at age 29? I never would have.