Hi P.,
What a hard thing to witness.... I'm so sorry.
Not to sound cold or clinical, but it sounds like you have been re-traumatized. Sometimes, even when we've done a LOT of work around some painful areas/events in our lives, we get surprised with feelings we thought were gone. Some things- random things we can least expect--can trigger us deeply.
Yes, it's happened to me, both as a very violent emotional response (not that I became violent, it was that the experience of being re-traumatized felt violently scary) and as long-term experience elicited by continued interactions with a person over a period of time. The former- that sudden, shocking feeling- it emotionally hijacked me for a short time. I had a pretty profound terror response and needed to go back to talk with my counselor about processing this, separating fear from reality (which I already knew but needed reassurance of) -- that one moment happened quite a few years ago but I won't ever forget it. The longer exposure was more on the 'annoying and concerning and deeply frustrating' side, but had a very significant consequence on my present day's life choices-- in examining this longer, more drawn out re-traumatization, I decided to choose a different path for myself than one I'd been on for a long, long time. Frankly, I am really grateful that I had the room to make that decision, and my husband has been very supportive.
Sometimes those chemicals in the brain just sort of hijack us in the moment and it is such an overwhelming feeling of helplessness. You could identify with this poor little boy because you had once been that sweet, scared little girl and have a sense-memory of it. You are likely at peace with this part of yourself for the most part, but there is the chance that you could be triggered at times. I know that working through my own past (which includes a PTSD diagnosis for events from my childhood) was helpful overall, and along with cognitive behavioral therapy, significantly reduced the power of most triggers to get to me in the present day. It's like building a good suit of armor that's healthy to wear and light on the wearer-- but there may be chinks in the armor, here and there, too.
Remember how much you have overcome in your life, how you have not let your past hold you back, but have striven to create a good life for yourself and your family in the present. Loving ourselves wholly, even as flawed individuals (hey, I still bite my nails from time to time!), is a day by day exercise. How much empathy and love must you have had-- not only for that little boy, but also for that sweet, helpless girl you once were, too. Loving and caring for those hurt aspects inside ourselves is brave-- you are feeling it, not shutting it down. The mere fact that you were able to then listen to the mother without judgment, without having an anger reaction at her-- it's probably the nicest thing interaction she's had in a while. She is hurting and likely feeling very sad and helpless for her kid as well. She may have very low coping skills right now. You didn't judge her, you just listened-- not everyone who has a response like yours would have been able to give that to her. You did a beautiful thing, P..
I'd encourage you that if you are still feeling troubled about this, to go and talk to someone you trust or a good counselor. I know these responses can leave us feeling shaken and unsettled. Know that you will feel back to normal in a few days as the chemicals which come from these feelings dissipate and your usual body chemistry kicks in.
sorry this is so lengthy... but, many hugs to you. I wish you peace and for you to feel your own strength as someone who has endured so much and come out of it as a good mom and better person. Be kind to yourself in the coming days and know that so many of us have these reactions-- you did beautifully.