Why would she ignore me?

Okay so me and my girlfriend have been going out for a bit over four months now. I sent her a text on Saturday because I wanted to talk to her and it had been almost a week since I had. She never replied so I thought maybe she was just busy or whatever since I did send it around 7 at night. But she never texted back at all the next day and I'm pretty sure she saw it since she was on Facebook. We text and use Facebook both to talk to each other sometimes. She did text me today but that was just to let me know something was canceled and she was pretty blunt so I know she saw I texted her by then, but never mentioned it? I texted her saying "thanks" but she never replied back so I just don't know what's up? I mean I'm in no way clingy or anything, and we really don't talk or she each other all that often anyway. I just think it's a bit unfair to me because I never ignore her or anything when she wants to talk. So I'm curious if anyone could give me an idea as to why she does this?