Orgasms from making out

Timber

I’m a virgin in every way except kissing. I have my first boyfriend now but we haven’t been dating for that long, it has been about a month. Anyways we were kissing and I was kinda straddling him and I felt something happening to myself down there and all of a sudden I felt a burst. I masturbate so I know what an orgasm feels like but I’m very confused as to why and how that happened. Bc its not like we were really rubbing/grinding or anything. And it happened not just once but twice. And I feel like this isn’t a normal thing... can someone please explain or help me out?