I hear this around our house more than I would like. It's replaced "Why?" as Paige's phrase of choice. As she gets a bit older she's decided that she can do more things All by myself.
Time to take a shower? She can do it all by herself.
Time to go to school? She can do it all by herself.
Anything that can be done can be done all by herself. If you try to help her, she won't like it. You must let Paige try it first. If she can't do it, she'll let you know. Two seconds ago it was "All by myself." Now it's "Mommmmmmyyyyyyyy, heeeeelllllp."
At least the why stage was short lived.