Damn man. Nothing happens out of the blue like that. She didn't just wake up one day and think to herself "wow, I don't know if I'm still in love with my boyfriend or not". Either this is a feeling that she has been ignoring for a long time because of 'being comfortable' in an established relationship, or.... or there actually IS someone else. Remember with texting and the internet available emotional relationships are easy to start and maintain without anyone being the wiser. Regardless of the reason that she feels the way she does I can empathize with the notion of loving someone, but not being in love with them. It sounds bad no matter how you say it, but is basically losing romantic feelings for someone but retaining the friendship feelings. Maybe you should find out exactly why this came about. If she doesn't want to talk for too much longer you might as well consider it a breakup.
...one can be sure of nothing until it has already happened...