Have you tried to understand if she feels anxious when she's without her phone, or if it's really just inattention? Because there's a big difference between someone who's addicted to social media and someone who's genuinely stressed about work or family.
The conversation needs to be really specific - it's not about complaining that she's always on her phone, but rather setting concrete moments where you both agree that time is just for the two of you. Like, "let's have dinner without our phones" or "when we're watching a movie, leave your phone in the other room". The thing is, she needs to agree and actually commit to it, otherwise it's just empty talk. If she says she can't disconnect for even two hours, then yeah, that's a warning sign that something's wrong - either she's really overwhelmed with work (and then you two need to talk about that) or she genuinely has her interest somewhere else.
But be careful about one thing: sometimes we confuse "she's on her phone" with "she doesn't care about me", when actually she's anxious, depressed, or so buried in work that she can't switch off. Before you conclude that she's lost interest, ask her directly how she's feeling, if everything's okay, if she's stressed. It could be that her behavior has nothing to do with you, but she's not telling you about it because she thought it was something minor.