kunatta, what is that mean?

I came across an example today, I don't really get it. Can anyone explain it to me?

The phrase is like this:"朝は　寒くなかったけど、夜は　さむくなったね。”

I do understand the first phrase, but the kunatta in the back confused me a bit. And I try to translated it and it became:" It's becoming cold at night". I just want to ask where is the kunatta came from and what does it mean?