The more important question is "Who told Lisa about the couch?" Lisa should be able to piece things together with what she has..
Whisp, when Lisa told Tommy she heard he and Misty were on the the couch, it sounded to me like she didn't believe or didn't want to believe what she had heard. To me, it sounded like Lisa wanting to hear Tommy say that it wasn't true. Tommy stifling her in mid-sentence the way he did makes me think that whatever it was, it's true.