The thing is, Sirius thinks later, is that he'd taken a lot of shit up to this point.
He doesn't tell the others, of course, but his family is large and influential and what this means is that Sirius always has a cousin or two hanging around at Hogwarts. Sure, the worst of them have left, but there are the sneaky relatives left behind - like his little brother, though Regulus is smart enough to stay away most of the time - and they do like to rile him up.
Usually he can ignore it, but they step it up in the spring for some unfathomable reason and it's enough to have him on edge for a few weeks.
Remus - Moony - is the one who comments on it. "Are you okay?" he asks one day as they sit together, trying to work on the map.
Sirius shrugs and doesn't answer. Moony sighs and Sirius presses his lips together tight - this isn't Moony's problem; it's not anyone's problem but his own. He'd tell them, he thinks, if it wasn't a family thing. He might not want to be a Black through-and-through - he might not like their philosophy or their prejudices, but keeping family problems in the family is a strong pillar of his personality. He can't just tear it down now.
He waits a minute and grins. It's not perfect and he knows Moony sees right through it - the rolled eyes are a dead giveaway - but he smiles back anyway and it turns Sirius' a little more genuine, even as he feels something flip over in his stomach.
Later, he's sure that he wouldn't have done it if they hadn't been taunting him about the others - specifically, about Remus.
No one knows his secret, of course; Sirius suspects that somewhere in his family, someone knows, but Moony isn't important enough to them for them to spill it.
Important to them. They know he's important to Sirius. Sirius rather thinks that's the point.
So when Snivellus makes some thinly-veiled comment about Sirius and Remus sneaking off into the woods, Sirius feels a streak of hot anger that he hasn't felt in a long time. (No point feeling it at home where apathy is the name of the game.) He smirks a little, lets his head tilt in that presumptive Black manner and gives him enough subtle hints that he knows Severus will be running for the Shrieking Shack by the time the moon comes up.
Another thought that occurs much, much later: if he'd thought he was doing right, he probably wouldn't have felt sick as he turned on his heel.
If he'd thought he was doing right, he probably wouldn't have told James.
He slinks around the castle grounds, spends a night out in the rain and wonders just how long he can avoid going back into their room. The thought makes him flatten his ears back in distress.
None of them have come to find him - not even James - and that, more than anything, tells him how badly he's fucked up this time.
You were bound to eventually, he thinks darkly, the voice in his head oddly reminiscent of his mother's low, dangerous tone. A Black in Gryffindor? It was only a matter of time before they discovered your cruelty, your madness-
Night falls and he shakes himself off, aware that he's sulking and he needs to apologise. It's not about him, not this time.
He changes back before he steps back inside the castle and uses secret corridors to reach the Gryffindor common room - no point in being spotted by anyone else, after all. The Fat Lady stares at him but says nothing, swinging open when he forces the password out in a hoarse voice. He has no idea how he looks.
The common room is empty of everyone except James and Peter, who are sitting by the fire, parchment strewn all over the sofa between them. They both look up when he stumbles in and Peter lets out a high, surprised noise. James purses his lips but doesn't say anything for a long moment.
Finally, he says, "He's upstairs," and he turns back to his work without another glance.
Sirius swallows around the lump in his throat as Peter looks away too - though he looks distinctly guiltier about it. Sirius walks to the stairs - and then takes them two at a time, figuring that the sooner this is done, it's done. Putting it off isn't achieving anything.
"Remus?" He stops in the door, his heart hammering.
Remus doesn't look up. He's reading a book - well, the book is open and in his lap, but he seems to be staring beyond the pages, at something only he can see.
"Remus, I- I'm sorry, okay? I didn't mean to- I didn't think-"
The book closes with a snap that echoes around the room. Sirius flinches.
"No," Remus says lowly. "You didn't think. You never think! Do you know what I could have- What I would have done to him? I know you don't like Severus, but do you really want him torn to pieces? You want me to do that?"
"No, of course not-"
"That's what would have happened, though!" Remus all but shouts and when he looks at Sirius his eyes are burning. "If James hadn't grabbed him and then seen me off- I didn't know him. I didn't know James. You put everyone in danger and for what?"
"I'm sorry-"
"I don't care. I honestly can't work out what hurts me more right now, Sirius - that you put Severus in that position, that you put James and I in that position, that you gave away what I really am... You know how I feel about this - about all of it. You were the one person I expected to never use what I am against me."
His voice is quieter now but there's anger threaded through it and Sirius realises with a dawning horror that this is by far the worst argument they've ever had and he may never be able to fix this one mistake. Maybe it would serve him right, but the thought of losing Remus, for any reason, makes his stomach turn.
Remus sighs eventually and he looks tired, so tired. He's still angry, but Sirius can see he's disappointed, too. Remus expected more of him. Sirius can't give him that.
"Just, leave me alone Sirius, okay?" Remus doesn't look at him. He reaches with a clumsy hand for the book he dropped on the bed. He looks kind of sad and tired and incredibly alone.
Sirius makes a small, pained noise in the back of his throat before he turns on his heel, running back the way he came. James lifts his head when he rushes through the common room and says something - Sirius thinks it's his name - but he's out of there before he can really register it.
He wanders the corridors until he looks outside and notices streaks of light in the sky.
He has to find a way to apologise to Remus. They have to be friends again.
They have to.