why does my boyfriend(27M) lightly slaps me (20F) while making out?

we've been together for over 2 months now and two days ago we were making out in his car ( that was my first time ever making out with a guy) and suddenly he would gently slap my face while shoking me , he kept asking if i felt comfortable and if he should stop and i told him no. i dont really know what it means or if it's a red flag to consider . an other thing is he stuck his thumb in my mouth while kissing my neck . i was kind of surprised as it being my first time and didnt know what to think about it.

UPDATE: so a lot of the comments think he just dove right into it ; as i mentionned in the comments he did ask if i was okay with it in the beginning before starting anything. it wasn't real hard slaps, just little taps ..