Tell the fat girl to forget off and leave you and your girlfriend alone.
She should have realized, or found out from someone, that you were dating someone else prior.You could try to cheer her up, and still basically tell her that you do not wish to date/go out.
Instead of being a jackass like other people suggested to you, just explain it to her.
She knew i was going it with somone already because my girlfriend is good friends with her.
"Are you being loving serious???"
This happened to me once, since i took a while to say no, my girlfriend ran of crying aswell as the one who asked me out :/
.And she just walked off looking like she was about to cry.