Women are more unsettled when they perceive a discretion has occurred in relation to their partner's emotional fidelity.
Men are more unsettled when they perceive a discretion has occurred in relation to their partner's sexual fidelity.
All this is based on sexually divergent motives between the genders in regard to relationships that have been developed via evolution.
Women want emotional fidelity so that the man stays around to provide resources to help raise children.
Men want sexual fidelity so that they are sure that the children that they are investing in are their children.
She most likely did this because one of her friends saw you flirting with another woman (act of emotional infidelity) when she was away at school.
Being that men do not value emotional fidelity as much, you most likely do not remember the incident in which you flirted with this other woman.
But she was hurt by the rumors so she felt compelled to reciprocate the emotional injury by making you suspect that she is putting herself in situation where sexual infidelity may occur.
Each gender somewhat implicitly understands what types of infidelity more affects the other gender but neither really understands the extent to which each type of perceived infidelity causes emotional distress.
Because you are both young and she did not want to admit that she was hurt by the rumors, she did not initiate a conversation about her feelings and opted to reciprocate emotional injury.
Before you also over react, I suggest you productively initiate a conversation about why she decided to act in a manner that seems to be designed to make you feel insecure.
BTW, I am 30 yrs old and also use "Dave" as my pseudonym.