Peter was actually rather angry himself, angry that his friends had been keeping secrets, as if he wasn't trustworthy, or as if he wasn't worth telling things to in the first place. Except where Sirius got riled up, Peter just got sad, and was feeling pretty low by that point. He sat, slouched a bit on the bed, messy hair (still a bit purple) hanging in his face.
"Course we want to know. When have we ever kept secrets from each other before?" That was the big problem in his mind. As soon as they started keeping secrets from each other, things would only go downhill from there. And Peter, he told his friends everything! Even embarrassing stuff, like the time he had gotten locked in the loo for an hour.
"Yeah. Without us," he chimed in with a nod of his head. "That's not nice, not at all. Some friends," he said bitterly.