((Okay. Just an idea here, tell me what you think. When they get to the scene and Sally says what she says, Sherlock gets suddenly defensive and ends up wondring why, getting shocked. Then when Lestrade asks who John is, again gets randomly defensive. During the crime scene, Lestrade flirts with John (or tries to) and possibly John flirts with him, Sherlock suddenly gets the first minute sense of jealousy and then becomes cocky as hell and desperate to show off with his deduction of the victim and all that jazz.
It's too soon to have him overly jealous or defensive, but because John is suddenly in his life; he starts to pick up bizarre feelings and questioning himself for the first time as well as the factor of John. Then of course pushing these of saying he's not feeling anything at all. And