When someone breaks things off with you abruptly what does that mean?

I didn't do anything wrong I was trying to take things slow, not get physical too fast, and one day she started to ignore me and totally just gave me the cold shoulder. My ex never explained why we broke up, but this is tearing me into pieces I feel like I can't go on with life. I feel helpless, lost, and confused...why did she do this to me? I don't mean to sound self-centered but I was a good girlfriend I don't get it, we were both laughing and smiling all the time. can you guys try and get a sense of what might've happened?