You likely qualify for British citizenship, provided your mother was not required to relinquish her British nationality as part of the creation of Canadian nationality in 1947. (How would you know this? Well, the easiest way is if she has possessed a UK passport since 1947.)
If she didn’t relinquish her citizenship, then you are a British citizen. However, because of the changing nature of British nationality law, you will have to first register as a British citizen before applying for your passport.