I'd say it's a movie from the 80's or 90's.
At one point the main character sees a monster in the street (I'd say at night, in an alley). It's quite a big monster and the monster can talk. They talk and the guy is frightened and the monster agrees to spare his life under one condition: that the man never ever talks to anyone about the monster. So the man agrees.
Then life goes on. He meets a girl and they have kids (if I recall correctly, not 100% sure about the kids).
But then the guy is not doing well: he thinks about that monster he saw years ago all too often. Eventually after years of silence (I think years went by because I think they did have kids), he breaks the deal and talks about the monster to his friend / wife. And the wife starts screaming / yelling at him and... she turns back into the monster.
Not too sure about what happen next.