Huh ??
It was really, really big and, yes, sorta-square rather than 'round' ??
Aha !!
Some years ago, exactly the same happened: A too-big, sorta-rectangular Moon, and in the 'wrong' part of sky ?
Took me rather too long to figure that. Did better this time.
Neighbour's big upstairs window near that corner is double glazed, and this evening's chill had shrunk gas, made it a concave, front-surface mirror.
Sorry, by time I'd grabbed camera, cloud cover had concealed this rare illusion of a square Moon.