Well. It was not an emergency but for some reason oxygen was released. I guess the plane was simulating passengers using oxygen masks. I don’t know why it happened. The red light CABIN ALT HI came up during climb around 12,000-13,000 and remained for the rest of the flight until I descended for landing.
In the next flight CABIN ALT HI light came up much later, above 30,000 and oxygen also started to flow but at much slower rate.