Remember when you first met and every day was peppered with a constant exchange of flirty, borderline-salacious text messages? It's normal for your text game to fall off from that initial honeymoon phase, but if she takes hours to respond without explanation, and can't muster up an ounce of enthusiasm when she does, it's not a good look for your relationship.
There's a real possibility she's under stress at work, or just in an emotional rut. But if you're able to rule that out (which you can do with a few simple questions) and you notice she rarely takes the initiative to text—and if she does, it's to ask where you are—she might be tracking your whereabouts for less than innocent reasons. We hate to be the ones to say it, but she's not suddenly interested in the minutiae of your day—she's covering her ass.