He is cheating on you, but he thinks you don't know, but you already know but your just playing along, you are At your friends house and Tom is in bed with a random that he is cheating on you with. The random girl looks at Tom and she says

"she knows.."

tom chuckles a little as he says

"No, she doesn't I'm keeping us a secret"

The random girl smiles at Tom as She is only listening to tom