Is it normal to be jealous of your friend's girlfriend?

I have a friend. I'm going to call him Dean.
So Dean and I have been friends for over a year now. I was in love with him all throughout last year. Last year we went out on a sort-of date (I'm not sure if he meant it as a date or not- he offered to pay, but later asked me to pay him back), and we stopped talking for a while over the summer. However, a little bit after school started up again, he and I reconnected. We're so much closer now than ever before, but things have been weird lately.
You see, Dean has a girlfriend (whom I'm going to call Jenny). He and Jenny have been dating since late October. She's a grade below us. He seems to really be in love with her, and vice versa.
I don't think I have feelings for him anymore. He's more like a brother now. But lately whenever he talks about Jenny, I get annoyed and upset. I try not to show it, but he asked me a few weeks ago if it bothered me when he talked about her. I said, "Sometimes," and he said he tries not to talk about her to me. Since then, it's almost as if he talks about her more! It's awful. And the jealousy just keeps growing. I feel so stupid when I feel jealous, because I'm not even sure if I like him that way. Is it normal to feel jealous of her?