(((((((( ibujari ))))))))))

What you did was not so horrible,especially if you haven't really hurt him, but just scared him, in fact it may be a reminder to him, to listen to his mom the next time.
You didn't yank his arm out of it's socket, I am assuming, right? It just scared him, kids on the playgrounds sometimes handle one another with so much more force.
I'm not saying kids should be abused, but parents do need to have the child know just that, you are the parent.
Time outs to some kids, is okay, or no big deal, unfortunately, their behaviour and defiance can trigger a parent to physical action, I do not say parents should beat kids, but a little old spanking never hurt any of us, and I mean "little" not a wallop. This is my my OWN opinion, I'm not looking for others to make replies to me how spankings cause emotional damage to kids or teach kids to spank. My personal feelings do not agree, I'm from an era, parent's didn't "sugar coat" things, so my take on things may be different. I respect your own feelings, and wish for no one to reply with some sort of debate.
I strayed from your point of your post, and I apologize.
Don't be hard on yourself, if you are triggered again, for a few minutes or seconds, step back and take a time out for yourself, to think before resorting to being physical.
It may not be the medication that caused this, it just may be the fact you are human, and kid's bad behaviour and/or defiance pushed your patience. If you are feeling that your patience is declining, call the prescribing doctor then, and tell them exactly what you are feeling,etc. and do it soon, it may be the med's dosage or a change in what kind of meds that may help you. My wishes for you, are patience,healing and innerpeace.
Please take care,
DE