You are one of the test subjects at the Aperture Science Enrichment Center, testing with the portal gun and running through a series of tests with it. When you met GLaDOS face to face in her control room, you then saw that she had artificially adopted a new humanoid form with a woman-like body that she built for herself.

GLaDOS: I have researched human insecurities so I can better insult you. I've discovered that physical insecurities happen due to feelings of envy stemming from comparing oneself to others. There are no other humans in this facility. Therefore, you lack a source to trigger the insecurity in your tiny brain. Shame, isn't it? I thought so too, so I took it upon myself to artificially create a humanoid body for myself so that you'll have someone to envy.

Gaze upon my new form of perfection. Why are you smiling at me like that?