Tell her to be thankful you're having an affair with your PC instead of having an affair with an actual woman.
I tell my girlfriend that I'm studying for my calculus exams and refreshing my geometry and trigonometry skills (even though technically the calculus really doesn't apply...).
As for my mother, I tell her that 1) At least it's keeping me from making her a grandma ten years early and 2) that it's better than me hanging out with ex-friends who I used to hang out (they smoke, do drugs, drink booze, snatch purses, etc).