Lily had gone past the stage of being upset and was now angry. She was angry at herself for not confronting Sirius earlier, angry at him for betraying her, and angry at Annette Parker for being a bitch.
During the last few days James Potter had become her saviour since she had sobbed into his arms, and told him what she had seen. He had also been the one to convince her that hexing Sirius's balls off, and not to 'mangle her pretty little face' was a bad idea.
Sirius, however, was increasingly worried about the amount of time Lily was spending with his best friend. He had tried to persuade them to get along before to no success, and now they were acting as if they were best friends.
As Sirius walked into the common room he saw James and Lily huddled into a corner leaning forward and talking in whispers.
James looked up and smiled at him, but he did not return the gesture. Instead his face remained blank as he walked over to where they were sat.
"Hey mate, you alright?" James asked. He knew that Sirius was not alright from the expression on his face, but didn't want to put him in a worse mood than he already was.
"Hi," turning to Lily, "Can I talk to you?"
"Sure." She said getting up and following Sirius to a deserted spot in the room.
"What's going on with you and James?" he demanded.
"Nothing Sirius, James and I have become friends, that's all."
"That's it? That's all there is to it?"
"Yes, that's all there is to it Sirius. Why did you think that we were shagging or something?" she paused. "You did, didn't you?" she hissed at the expression on his face.
"What the hell was I suppose to think?" Sirius yelled.
"Well what the hell am I supposed to think about you shagging Annette Parker?" Lily yelled back.
The whole room was not silently watching the exchange, wanting to know why the golden couple were arguing.
Sirius's face looked shocked as Lily revealed that she knew about him cheating.
"Lily, whatever you've heard, it isn't true. It's all lies." Sirius said steadily trying to reassure her.
"I haven't heard anything. I saw you together." Lily said calmly.
"Lily-"
"Don't bother Sirius; I don't want to hear it. It's over." Lily said interrupting him and walked up the stairs to the girls dormitories.