[Heaven forbid you expect anyone to be attuned to their body and senses, Sebastian thinks - very charmingly - to himself. But most likely that does mean it's only the strength, yes? He certainly feels different in more ways than sudden physical weakness, so if it were some kind of complete switch...
Speaking of weakness, however... Sebastian stops walking when he feels himself swaying a little. Ugh, now it's starting with that slight light-headedness he now knows to associate with bloodloss (and how undignified isn't that, someone like him having felt feel weak from something so trivial as a little bleeding, not only once, but twice now?).]