if not seeing her for a week made u forget about her, she is probably just that- forgettable. absence usually makes the heart fonder, not colder. lol. although you may just be saying "easy 2 4get about her" because you are upset about her hooking up with someone else....

you could always try talking to her. maybe she made a mistake hooking up with her ex. maybe she regrets it. maybe she was too drunk to know what was happening...etc. And if not- she is not worth pursuing.