Why is my girl friend so upset........? HELP >:l.?

So the other day, we were having (we've had before). Things were heating up, and I moved her hands down to my boxers. Right when I did that, she pushed me away, got teary eyed and said " I have to go" .. That happened on Friday and she won't answer my calls or anything. I tried visiting her but she won't open the door. What could be wrong?!?