I am sure it is a phase and is mainly due to her resently being around the staff at work who all (obviously) call me by my name.
I am trying to combat it by not answering when she calls me by it, and responding with 'I am Mummy' then addressing her request.
However I have to say its horrible to hear her say it over Mummy and she has also started shortening that to Mum which is not sitting well with me either. Don't get me wrong I am not a fan of children calling their mothers 'Mummy' beyond a certain age, and as an adult its just too sickly sweet but Tabitha is 2 years old.
It has reminded me that I used to have a friend whose children called her by her first name at her request, it never really occured to me to think it odd, but it is don't you think?