Is expiation obligatory for one who lied under oath to cover up past transgressions with her fiancé, from which she has repented?

If someone disobeys their Lord and commits a sin, they must repent and conceal their sin, not informing anyone. The questioner has done well by keeping what happened to herself. Lying is forbidden, and the yamin al-ghamūs (a false oath sworn deliberately to usurp the right of another) is fundamentally a grave sin. However, if there is a necessity for it, and it serves a legitimate Shariah objective, and dissimulation (tawriyah) is not possible, then there is no blame in it. Caution must be exercised in interactions between engaged couples to avoid falling into forbidden acts. The yamin al-ghamūs does not necessitate expiation (kaffarah) according to the more preponderant opinion, but expiation along with sincere repentance is more cautious and absolves one's responsibility more fully.