Is it normal that i can't get this off my mind?

A friend of mine named Lucy (names changed) introduced me to my current boyfriend, Derek. She was dating his buddy and employee and still is. I wasn't interested in him initially. Derek would say things about Lucy such as "She is so intelligent" and "her boyfriend is going to regret cheating on her, and losing her" I just got the general impression that he respected her and her friendship. But deep down I acknowledged that he wanted her but couldn't have her, as she was with his employee\buddy. After I started dating him my friend Lucy became more and more distant from me, to the point that it was awkward, like sometimes we'd go to their place and she wouldn't say a word to me. Then, at a work Christmas party Derek became the drunkest I've seen him, broke the lamp in the hotel and sat in his boxers as his bosses attempted to calm him, took his shirt off at Denny's, and told Lucy "Your my best friend and you won't be with me". When I ran off to the bathroom, overwhelmed by his behavior the entire night, he followed me. "Fuck off" I told him. He went into the lobby and broke the Dennys window, then got taken for a night in jail. Afterwards, I dwelled on it so much for so long, almost a year, that "Lucy" is a forbidden topic. He doesn't hang out with Lucy's boyfriend outside of work anymore, and he thinks its turned into something else with me beyond "Lucy". I know I can be too jealous and make things up in my head. But why do I get this sinking gut feeling and my heart feels like its being ripper out of my chest when my rationality becomes weak? I was always told to listen to your gut if you think a guy wants to cheat. I have no proof, but is it normal that I deeply hate this girl, and feel threatened by her even though she's practically non-existent in my life now? I wonder if its because Derek hasn't been honest with me. He denies ever having feelings for her, which goes against his behavior, words, and body language. Maybe if he could just admit he used to have feelings for her I could move on