How gullible is my gf?

My gf moved in with me a few months ago. Things are ok with us I guess, but she's not fully satisfying me. I spoke with a buddy of mine that lives a few doors down in our apartment complex. He introduced me to one of his girl friends who was more than willing to supplement what was lacking in my relationship with my gf. So heres the funny thing. I started telling my gf that I had to go into work every Sunday for a staff meeting, when in reality I was going over to this new girls house and well, you know. So here I am, its Sunday again, and I just told my gf that Im going to the staff meeting...lol. She'll believe anything. Makes me think what else I can get away with.