How likely is colon cancer in a 15 year old girl? My daughter keeps thinking she has it for some reason What about are the chances?

Very very rare. Other for congenital situations colon cancer under 30 yrs of age is rare. Under 20 years of age is even rarer. The polyoma virus that initiates many of the cases requires a series of steps including the development of telomerase to initiate the malignant transformation process with mutation in p53 and the APC gene taking place. This takes years and why colon cancer appears at middle age.
Rare. We never say never in medicine, but we do not see Colon Cancer in children. What are the reasons? What is she most worried about? What other family stressors have come up? Any deaths in the family recently? I would pursue this line of thinking with her pediatrician in a face to face visit.
Very unlikely. It is very unlikely that she has this. Did lshe get the idea because other members of the family had this. Is she having abdominal symptoms or intestinal symptoms or rectal bleeding or constipation. There are many other reasons besides colon cancer for her symptoms. This is something to discuss with her and her pediatrician, and if necessary a gastroenterologist.
Zebras and unicorns. The chances that she has colon cancer at age 15 are about as likely as seeing a zebra outside the zoo in the US. It isn't impossible (a unicorn) but without a family history of early cancer, a polyposis syndrome or some other factor, it would be a medical novelty. There are other tumors that occur in her age group of course, but these are not regular colon cancer.