'Cause Karma's gonna get you now
And I'm not gonna stick around
Karma's gonna get you now ow ow baby
And I know that I'll end up right
Derived from Indian religions, Karma is the fundamental belief that if one person does an action, another action will occur to balance it out. Popular Western interpretations of this are "reap what one sows", "violence begets violence" and "live by the sword, die by the sword".
Kristina is simply making the point that whatever this person's done against her, she's not going to stand by them or forgive them, nor will she take revenge; she's simply going to let Karma run it's course and balance out the actions ("end up right").