Why does my friend think I am jealous of her having a boyfriend?

Hi, so today, me and a couple of friends were hanging out, like we usually do. We ate lunch, watched a movie, and then I started to get out a book and read it. My friend was texting her boyfriend and saying "ILY" and other stuff like that. I peaked over her shoulder and said,"Awww." Then she turned around, gave me this angry expression and told me I was just jealous that she had a boyfriend and I didn't. Why would she say something like this, and how do I make it clear I am not?