Why does my (F18) boyfriend (M18) becomes distant after he finishes?

I (18F) have been dating my boyfriend (18M) for almost a year.

Yesterday, he came over, and we cuddled skin-to-skin , acting romantic and affectionate for like 3 hours,it was one of the most vulnerable moments we ever had. Eventually, we did something intimate.

After he finished, he cleaned me up, got dressed quickly, and then just laid on my bed and asked if we could watch tv. I put something on and he didn’t even try to talk to me,he felt really cold and distant.i asked if I could put my head on his shoulder because I needed touch and he told me no and just to “deal with it”,he didn’t even look in my direction for 40 minutes. After,It felt like he was trying to leave as soon as possible,because he called his sister three times to see if she could pick him up (she was nearby with friends and has a car, while he doesn’t have his license yet). Eventually, she picked him up.mind you it’s like 4 AM.

The fact that we were so so close physically and emotionally,and not even 10 minutes after He is cold like a stone and doesn’t even wanna look at me is so overwhelming for me,I mean, He usually like that after intimacy but it felt more severe this time

My question is: Why does he seem to lose interest in me so quickly? Is it something I did, or is this just how some guys act? We are each others firsts in everything so I don’t have any prior experience with this

TL;DR: My boyfriend and I (both 18) had intimacy, but right after, he became cold and distant. He barely spoke, refused any affection, and seemed eager to leave, calling his sister for a ride at 4 AM. He’s usually a bit distant after intimacy, but this time felt much worse. I don’t know if I did something wrong or if this is just how some guys act. Why does he seem to lose interest in me so quickly?