Are you for real!! A dead sheep... WTF ! Seriously... it's not her honey its you... you're not keeping her interested... she probably just find you very boring and as soon as she gets off... she just tries to get you away from her the easiest way she knows how. Maybe YOU should work on your technique!
you must stink at keeping her horny, boo you.. but if she masturbates I would tell her to stop.. because that effects the length of time she stays horny.