why does the weather gets cold so suddenly?

Answer 1

Mainly from colder air masses moving off an adjacent continent, or colder area.

These air masses, especially those that only cross short stretches of water, slide under warmer air masses and drive the temperature down.

The "sea track" of air masses is especially important as, due to albedo, the sea tends to warm air masses and the land cool them ... though this is reversed at night (hence sea breezes reversing after the sun goes down)

Answer 2

Cold air from an adjacent colder area (like a part of the land that is near the ocean in the winter) can blow in on surprisingly-fast winds.