Fuck fuck fuck fuck fuck is kind of a great internal monologue. Severus is in no way embarrassed about Claire, and he wasn't hiding it from Lily, but what a way for that to get out. He still hasn't turned back around, concerned he looks like a tomato.

"I feel a bit like airlocking myself," he says, white-knuckle casual.

A cold accusatory feeling slithers up within him, suggesting wouldn't he like to speak that way to Lily anyway? But he recoils away from it mentally as if burned. No matter what anyone thought-- no matter what he can feel for her, if he was given room-- it's not like that. It's not some cheap unrequited romance.