Just a thought, you could end all this uncertanty now.
Pick up the phone, call her, and ask her right out if you scared her away with your feelings.
Maybe she thinks its too soon to be confessing your love for her, maybe she doesn't want a boyfriend right now and doesn't know how to tell you.
With the IM thing, the next time you are both online send her an email, tell her you are sorry for what you said, but tell her you just wanted to be honest with her and you didn't want to scare her or push her away, its just you really like her and didn't want to blow it.
Ask her to be honest with you too, ask her what she wants out of life, if she sees you in her future at all.
It can't hurt, and then at least you will know once and for all.