"creepy as hell?" okay please stop having sex now.
If your girlfriend is late on her period, it could be for multiple reasons, however one of the main reasons is that in fact she could be pregnant.
The only way to verify this pregnancy aside from waiting a few months to see if she continueally does not have periods and her stomach enlarges, is to infact take a test. Its not something that you can avoid, or "change your mind about". Although you may be unedcated on the idea of conception does not mean it does not happen to you. Whether your 15 or 50 you carry sperm that will inpregnate your unprotected girlfriend.
Please test, find out if she is, and take the proper steps from there.