Desvelada means 'sleepless' or 'awake all night'. It's a vivid word that goes beyond simply 'not sleeping' and implies a state of being kept awake, often by thoughts or emotions.
In the song, the singer uses it to express how the memory of someone's gaze keeps them awake: "Estoy desvelada, es por tu mirada / Que no dejo pensarla to'l mes" (I'm sleepless, it's because of your gaze / That I can't stop thinking about it all month). This word beautifully captures the intensity of their feelings and the impact this person has on them.
The title is a pun you only really see written down. Split it and it reads lo sientes [do you feel it], run it together and it is lo siento [I'm sorry], and Big Soto spends the whole song refusing to pick one. He opens by trying to delete the evidence: "Quiero borrar tu nombre de mi historial" [I want to erase your name from my history]. It does not work. Two lines later he is at a party doing the least subtle thing a person can do, "Estoy aquí en el party viendo el celular" [I'm here at the party looking at my phone], and the signal he is waiting on never arrives.
Then Natt Calma answers, and she is not letting him off. Her verse points straight at the pattern: "Siempre lo dejamos en el intento" [We always stop at just trying]. She has been up all night over it, "Estoy desvelada, es por tu mirada" [I'm sleepless, it's because of your gaze], while the hook keeps circling back to the same accusation, "No lo niegues, tú también lo sientes" [Don't deny it, you feel it too], repeated until it stops sounding like a question and starts sounding like a verdict.