That's probably approach avoidance ("autorejection") caused by moving slow and making her think in the back of her mind that you might reject her sexually. You can get out of it by disqualifying yourself; this is best done in person. Say you think you may be too much of a bad boy or something like that. She'll not really think you are a bad boy but depending on how deep she is into it her feelings will rapidly cycle and she will start using a foul language and will appear angry, but usually passes pretty quickly, and you have to remain calm and just listen. You might have to talk to her a bit and reason with her, but you'll get a window to escalate. If you miss it, you'll have a really rough time fixing things.
Vin Dicarlo wrote a short piece on it. I haven't tried his whole routine, but autorejection is really one of the hardest things to break:
http://dating-tips-online.blogspot.com/ ... ot-to.html
Seeing how you may be a virgin, you might want to think carefully before trying to score with her about whether you really want this to be your girl since you would probably get attached.