slight fever could be due to your nerves, causing you to have slight anxiety. to which a rise in body temperature will occur. then the level of dopamine increases. You might actually feel high and euphoric when you're infatuated with someone. In your brain the dopamine center is rewarded when you see or think about your love interest. Then your brain gets flooded with dopamine. This pleasure response feels so good that it's easy to mistake infatuation with a real connection.
The butterflies only comes from your first love, its innocent then. What follows is just scripted and I also subscribe to the thinking that true love only comes after marriage. When you get on your knee's and scrub her vomit from the toilet seat or when you can stand her when she's least attractive, that's real.