see the exact sentence that jassi said was that "aapne mujhe apni pataa maanaa isliye main aur kisike saath jaa sakthi thi"
what she means is that as i have been accepted as your wife (i solely take myself to be yours and yours only), i had the guts to be/stay with another man only as i am completely yours. In the sense that she was so pure in her love that nothing could tarnish her devotion to him. Anything could happen to her and she would not swerve from her dedication to armaan as she is his wife. (Its a very Indian concept right, once you are someone's wife you do not indulge even mentally with anyone)
i am picturising this dialogue as something seetha could have said to Ram before her agnipariksha
however, obviously armaan misunderstood what jassi was trying to say just like Ram misunderstood Seeta....ššš - his male ego could not contain himself and he slapped herš
Hope that helped.........