He knew Sirius was worried. His first instinct was to assure him that he was fine, or at least, that all was as expected. No sudden urges to maim, injure, or kill anyone. Just the familiar assault of heightened senses and a curiously strong instinct to protect the man in front of him, though they faced no opponent tonight.
He made his way over to him, his eye level with Sirius' waist, and nuzzled into his side gently. All things considered, he was feeling pretty content. Sirius' presence had something of a calming effect on him, even before he'd started taking the potion.