Like others, there is red flags all over this.

Had to do a double check to see if she was 18 because this is really immature behavior from her.

Seriously though, don't victim blame yourself. You didn't fuck up and you didn't do anything wrong. She should not be policing your thoughts and it was completely inappropriate of her to meet with your coworker.

After your inevitable breakup, Don't forget it's not wrong to be honest. Your sexual fantasy is nothing to be ashamed of, don't let anyone tell you otherwise.