It's to feel small and inferior, perhaps not proud of yourself.

It means you have done something so stupid that you feel so small.

It's not difficult to imagine being two feet tall, is it? How would you feel being so little? I'm serious! I would feel smaller than the smallest person in the world. I would feel insignificant, worthless, and that my life meant nothing. Now do you understand what it means to be about two feet tall?