It's certainly not unusual and you're definitely not alone.
I very occasionally smacked my eldest. But I was cross once and I was shouting, I raised my arms expressing iykwim and my ds1 flinched and cowered. 
I will never, ever, smack my children again.
On the occasion in question I wasn't going to smack him (he obvs didn't know that), and I've literally smacked him maybe give times in his life (he's 6 and extremely trying at times)
It upsets me now still that that was his reaction. And rightly so. Ill never forget his face.
I try to deal with anger/red mist/frustration differently now and it's rare I raise my voice now to my children.
My point is, what happened today may change your perspective on things. You now know(in hindsight) that your reaction may have been too hard. Take it as a lesson. And if you feel the need to, explain to your ds what happened and that you're sorry and it won't happen again, you love him very much etc.
It's hard being a mum sometimes and it's a learning curve. This is one of the tougher curves.