sexual advice please

To make a long story short I was sucking my potential boyfriends dick in the car and he got really into it by smacking my ass and whatnot. Then he just went for it and stuck his finger up my ass. I never had anything like that happen to me so it kinda caught me off guard, but after awhile of me still suckin him I start to orgasm from him fingering by ass. The question is: is that normal to have a orgasm from getting you ass finger fucked?