Rachel, your girlfriend, is secretly a balloon-girl. Someone who's body can inflate just like a balloon to huge sizes. But she only inflates when very happy, and tries hard to keep that a secret. However she was so happy at the end of your date night, that she couldn't help but begin to inflate. It starts out slow, stretching out her dress, but she knows she needs to get home fast before it is too embarrassing. She inflates very slowly, but by the end she will be massive and immobile