My girlfriend rarely replies to texts?

I m in a long-distance relationship with a girl and she just hasn t been replying recently. I always have to start the conversation and when I do, we always end up talking about whatever she wants. And if I change the subject, she just stops replying altogether. We ve been together for almost a year and a half now I feel like she s just ignoring me. And when I ask about it, she says she had to help her mum or something (which is a lie because I can see she s been on Twitter or Facebook). So what s up? Is she not interested in me anymore or what? Thank you :)