It could be that she is afraid of what could come of the relationship or is afraid of her feelings for you.
He upset her somehow?
You haven't asked her? How would anyone on here know?
The guy has probably said something that she finds offensive or has had enough of his behaviour or the way she has been treated.
Because she found out that he's an a'sshole or her heart and head were never really in it.
maybe she thought he was too cheap, a dime a dozen.
Maybe its that time of the month.