I was told it was very offensive to the Canadians because they too are Americans.
Did you ask the person advising you what you should refer to yourself as then?
I agree that many forget or do not think about the 'Americas' and all that they contain but no other country in the Americas calls themselves that - we are 'The United States of America'. They can even look at the money in their pockets, says it right on there.
When Canada changes their name to 'The Canadian Provinces of America', I'll be sure to clarify which 'American' I am...