Hi OP, I'm replying because your situation sounds really familiar to me, except that I would be in the girl's position.

Sometimes when things get really stressful I get in moods where I only want to talk to certain people or I can only focus on speaking to one person, so I only log on to one messenger. That, or I go invisible if not stay offline entirely.

She could be just having a rough time. Of course I'm only putting this up as a possibility, since I don't know anything about what this girl is like. I think >>4 has the right idea, but above everything else please try not to worry yourself about it.