I am this way too. I have depression and anxiety. When I'm depressed and/or stressed I withdraw. I'm not trying to ignore people, I just get inside my head and don't notice people. I found out a while back that people were talking about me, not being friendly. I felt bad because I don't want to appear that way. But I can't just snap out of it.
Just give them space, and just speak when they do. You will eventually figure out if they have a real problem or not.