Mary turns back to the house, fuming with anger, no matter how irrational that anger may be. Somewhere, deep inside her, she realizes that Rob had every right to be afraid. Every right to run away as fast as he could. Part of her knows that SHE had no right to come on to him like that, but most of her, the dominant part, is just angry over being spurned.
She's about to plot her vengeance when the sharp, stinging pain from before strikes back, and hard. Her spine arches back involuntarily as the pain comes not from her belly this time, but from her back, as a cow's tail erupts from her backside.
"Damn it..." Mary groans, agonized, "I thought I got rid of this crap..." And she did. But as she felt angry at being spurned by Rob, so was the ever-so-slowly emerging cow part of her - but it was she who had done the spurning. As far as it was concerned, it was just being created, and then it was going to be forced away? The cow-spirit had been placed inside her, and just as it was coming to the surface, it had forced back inside, and for that it was angry. It played nicely before, making the shift painless, but now it was going to play hardball, and Mary would suffer for it.