She did something rather...well..."impulsive". I bet she was provoked too! The girl she had that "incident" with is not doing toooo well so to cover it up and be safe she needs to get some of your DNA. And since she knows you and that girl had a little thing going, what better way to frame you for her crime than to smear your cum all over the crime scene? Of course she needs to get it out of you first...