Sorry, but I'm going to be a little blunt: She's lost interest in you.
Was it something you said or did? Most likely not. Women for the most part, if they are emotionally invested, will want you to figure out or know what you did that made them upset. Instead, this girl is pulling away. Granted, there is a chance that she could be a commitment-phobe, but I'm thinking that she just doesn't love you the way you love her.
It's hard and it sucks (one-sided loves always suck), but let's face it, this relationship just wasn't meant to be. Yet, knowing that doesn't make the hurt feelings go away, especially when you trusted her with your heart.
But what you can take comfort in is that there is someone out there that will relish your company, that won't be able to wait until you call or see them. She'll be the kind of girl that enjoys hearing your ideas and dreams, and supports you in them. Who you make laugh and feel protected... and most importantly, when you tell her that you love her, she will say "I love you, too" and mean it.
I am sorry that you had to go through this disappointment and hurt. It really sucks and my heart goes out to you.