*trigger warning* beating, rape and orgasm

This is long and graphic but I need help understanding. I was 13. They were 19 & 20. I was alone and they came in and grabbed me. I fought, I bit, I kicked and screamed. They hit me, one put his cigarette out on my arm. They held me down and took turns going down on me. They put something in me but I couldn't see at that point from the swelling and blood. Vaginal and anal. For some reason I had an orgasm but I Don't know why. I kept fighting until they knocked me out. I came to and fought but they had a gun and shoved it inside me and told me if i didn't shut the fuck up he'd pull the trigger. I passed out. I came to and one guy was thrusting in me and finished. They were done and one laughed and told me I would never be worth anything to anyone. He put himself back in me and he peed into my vagina. They left me there dying on the floor and there was so much blood I thought I was already dead. I don't know how long I stayed there bleeding but I finally crawled to the bathroom. My friend came home and helped me. I couldn't pee for 2 days and I don't think I've breathed since then. Out of all of that I just need help understanding why I had an orgasm. I didn't want any of that.