Tembló is the past tense of the verb temblar, which means "to tremble" or "to shake". It's a powerful word that creates a very physical image of a strong emotion.
In the song, the singer describes the moment he fell in love by saying, "Todo tembló dentro de mí" (Everything trembled inside of me). He uses this metaphor of an internal earthquake to express the overwhelming, full-body shock and excitement of love at first sight. It's a much more dramatic way to say you have butterflies!
Have you ever seen someone and felt like your whole world just shifted? That's exactly what Camila is singing about in "Todo Cambió." This song is a huge, romantic declaration of love at first sight. The singer describes their life as being in black and white until this person appeared, saying, "De blanco y negro a color me convertí" (I turned from black and white to color). It wasn't a slow process; it was instant and overwhelming. He admits it was "tan fácil quererte tanto, algo que no imaginaba" (so easy to love you so much, something I didn't imagine).
This isn't just a crush, though. The feeling is so strong that it feels like destiny. The singer says, "El universo escribió que fueras para mí" (The universe wrote that you were for me). There's a real urgency to make sure this person understands the depth of his feelings right now. He feels compelled to say, "Antes que pase más tiempo contigo, amor / Tengo que decir que eres el amor de mi vida" (Before more time passes with you, love / I have to say that you are the love of my life). It's a song about that one moment when you see someone and you just know.