First of all, Mstislav is in the extreme east of Belarus; just across the border from Russia. Secondly, that was four and a half hours ago. If this was a bombing run, we would have heard about it by now. Thirdly, even the Twitter post you cite claims that it could "again" be long-range bombers. Evidently, this has happened before. Fourthly, there are currently no air raid alerts in effect in Ukraine (except in Kharkiv oblast). Yeah, I'm monitoring this stuff.
Look, I understand we're all on edge, but there's currently no imminent reason to panic.