@galileogirl. You are correct that not all anemia evaluations (or other low blood count evaluations) are the same. Only a doctor who can take a detailed history and do a physical examination can sort out whether there are warning signs of a serious problem (such as yours) or not. Some of these would include jaundice (yellow skin and eyes), stomach pains (possible ulcer), vomiting blood (ulcer and other things), change in bowel habits, unexplained weight loss and fevers.
As for your diagnosis of colon cancer, you are fortunate to have had a persistent and curious doctor. However, it is quite possible that you actually had two issues, namely, iron deficiency from daily losses, AND, eventually, iron losses from gastrointestinal bleeding accompanying your cancer.