If my dh walloped me with a pillow whilst we were happily messing about, then that would be fine. If he did it in the middle of an argument, then he's doing it in anger, and that's not fine.
Before DV erupts into painful violence there is a lot of boundary crossing in the build up. Something to watch out for is poking. If during an argument your other half pokes you in anger, it's a bad sign. It's blurring boundaries and building up your tolerance to being touched during an argument.
This could be a one off or it could be an escalation. I wouldn't want to bet on which it is, I would be putting some firm boundaries in place. I would be expecting remorse, not someone justifying their actions.
There are lots of violent actions that don't physically hurt. He could smash something of yours. He could punch a wall next to you. He could spit at you.
Pain isn't an indicator.
Make it very clear that he has crossed a boundary, and that if it happens again you will be treating it as an act of violence (since now you're being really clear with him that that's how you see it, he can't pretend not to know in the future).
Sounds like the communication between you needs a lot of work. I hope you find a way to sort it all.