Just why Sybill Trelawney panicked Sirius so is hard to say - he had gotten the idea somewhere and it had taken hold in his fertile overactive brain that she had a thing for Remus, and he was protecting his friend from her - or somehow that is how he perceived it. The truth of the matter was a bit different, but Sirius was in no position to accept that truth yet. He watched the confused redhead as she lumbered out of sight, sighing in relief, his breath warm against Remus' neck, unwilling to move, or to disentangle himself from him quite yet.
"Mmhmm," he murmured, "maybe she'll fall into the Loch, her and her bloody crystal ball." His body was sending him all the wrong signals and his brain was starting to tell him that perhaps he should move away - but he wasn't listening, not at the moment.