It is possible that Canada may recognize you as an expat citizen due to your mother's citizenship already; however, your claim would be stronger if you had been born in Canada.
On the other hand, you have a better chance asking Canada to recognize you as a citizen without jeopardizing your US citizenship, than you would have were it the other way around.
The problem is that Canada recognizes the concept of dual citizenship. The US reluctantly acknowledges that other countries recognize the concept of dual citizenship.
My case was the opposite of yours; I was born in Canada to a Canadian father and American mother. The US recognized me as an expat citizen based on my mother's citizenship; Canada recognized me as a citizen because I was born there. So you may already have dual citizenship without knowing it. But either way, contact the Canadian Consulate in the nearest large city to you, or the Canadian Embassy in Washington DC. They were of great help to me in sorting out my mixed relationships to my two countries.