If the colonoscope detects abnormal growth in my rectum, does that mean i have colon-rectal cancer?

Uncertain: Any abnormal growth seen at the time of colonoscopy requires that a biopsy be done to clarify exactly what that "abnormal growth" actually is. The biopsy should give sufficient information to determine what the next step should be, ie, repeat colonoscopy, surgery, further testing such as endorectal ultrasound, etc.

Not necessarily: Any polyp benign or malignant is abnormal growth. Should be biopsied to determine what it is and seek appropriate intervention.