"¿De Dónde Sacas Eso?" (Where do you get that from?) is HA-ASH telling an ex that he has it all wrong, while giving away just enough to make you doubt her. She lists the evidence he is using against her: she asked his friends how he is, she keeps turning up at places where she thinks he will be, she spends the day checking her phone, convinced he will call any minute. Her answer every time is "No por eso creas que aún te quiero" (don't think that means I still love you).
The chorus is pure sarcasm. If it makes him feel better to think "que aún me muero por ti" (that I'm still dying for you), "pues sigue así" (well, keep it up). She even tells him "Si eso aumenta tu ego, sigue creyéndolo" (if that boosts your ego, keep believing it) and calls his idea "tu cuento" (your story, your fairy tale). She also has a theory about him: he is with someone new "solo por aparentar" (just to keep up appearances), and "Ya estás grande para esos juegos" (you're too old for those games). The fun part is that the more she protests, the less convincing she sounds.