Is my jealousy irrational?

I recently started dating this guy who’s a good deal older than me. Being a lot older, he’s also a lot more experienced. I’m 18, a virgin, and had never even had a boyfriend before him. I also told him before we started dating that I could be a very jealous person. He recently told me that him and a girl that he was sleeping with just a week or two before we first met are still friends and talk often, and naturally I got a bit jealous. He got upset with me for being jealous even though in my eyes my reaction wasn’t even irrational or unreasonable at all, I just got quiet when he told me like I usually do when I’m upset. Am I in the wrong here?
Is my jealousy irrational?
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