There have been lots of Queen Mothers through history though, not just 'the' one. There was a queen mother in 'the man in the iron mask', made leonardo de caprio quite weepy.
The term queen mother is not an official title per se, rather a description. Queen Elizabeth adopted it as part of her title to distinguish herself from her daughter. There were three queen mothers in the last century, Queens Alexandra, Mary and Elizabeth. You have to go back to 1669 to find the previous one.
There has only been one 'queen mother' as the mother of the present queen invented the term. Previously the widow of a King was known as a 'dowager queen'.
Not so, dowager queen is the widow of a king. In addition she may also be the queen mother of the reigning sovereign. The term is much older than the last Queen Mother.