We slapped our kids (on legs/bottoms) when they were little. Maybe 3-4 times each in total between us. (Me and DH both ... same approach.) So not that often, but it was useful sometimes. Always the ultimate sanction for bad/dangerous behaviour.
In our defence, we lived in various countries where smacking was allowed/encouraged ... & is probably still allowed/encouraged 2day!
Plus our parents smacked us when necessary when we were youngsters ... in fact everyone's parents did it! And the teachers smacked the naughty kids at school! So it seemed completely 'normal' to us ... & we didnt think we suffered unduly.
We also had 3 DCs very close in age and sometimes it was really hard to control them when they were fighting, doing dangerous stuff!
I do now sort of regret it ... as society has made us feel so guilty. Once I slapped DS1's leg out of anger/frustration ... which I know was wrong of me. But, the other times, as long as we didnt maim the kids, or terrify them, what was so wrong?
OK, now it's called "assault" but all u want is for ur DCs to grow u ... hopefully into nice/respectful people? But is the opposite of "assault" perhaps not caring enough? Not being in control at all?
I know nowadays u have to be firm/rational without resorting to a smack. We just have to explain what they did wrong & the consequences. But 20 yrs ago, we hadnt been told all this and smacking was for when u didnt have time for all that nice talking stuff.
So I reckon that we/u r very much at the OK end of parenting. And without the benefit of supernanny with us at all times, we all get stressed/make small mistakes every day.
What u did to DS was really no big deal ... except to u/him. You wanted him to feel your shock at his behaviour & you were a bit taken by surprise/off balance. That's all.
If u want, u can try to talk about it to ur DS 2moro and say sorry if he's also sorry for calling u a bad name?
FYI, I said a sort of general "sorry" to my 3 (all adult now) kids .... sorry for smacking them a few times when they say between 2 & 7 yrs old. They all happily accepted my apology ... but they didnt care/remember much and thought it was fair enough that they got a few smacks ... some rough justice on occasion/the wrong perpetrator identified etc, but they accepted it as normal family discipline overall ... they deserved it ... & it was better than mental torture? Quick smack .... & it's done/over?
They have much more issue over other childhood 'stuff', eg why we never had a dog etc!
Anyay, if u still feel bad, just tell ur DS u love him and u didnt want to harm him with the smacking ... u just wanted to shake him a bit into seeing how shocked u were at his reaction?
U're a good mum! Hope u sleep well!