So a few days ago I went to the gym and was being dropped off, I didn't finish my breakfast (literally just a slice of bread) and so I put the slice of bread on the roof of my friend's car jokingly, we were both laughing and I was walking towards the entrance of the gym. Suddenly I hear intense pounding on the blacked out window next to me and hear a guy screaming at the top of his lungs and cursing at me about littering bread in his parking lot and to pick it up. I practice stoicism and try to be a guy that not much affects, but I was very caught off guard. I scoffed not being able to say anything, which only made the guy angrier. Eventually I did go back to my friends car, who was already about to turn onto the road, with the bread still on top of their car. I'd like to state that the bread never actually littered the parking lot, but trying not to escalate it further I picked it up and threw it away. I did the backwards "V" sign at the guy just to flip him off, then went and worked out. I forgot about it for a while but I'm now realizing, at 3 am, that whenever I think of it and think of things I wish I would've done (say no and walk away) I feel my heart literally pounding through my chest - just thinking about it. I'm ashamed to say this rattled me, and I now have intense anxiety about it with no rationale except that he maybe would've tried to beat me up or something. But still that's unlikely, and I was wondering if anyone had any advice or help for me to get past this, I'm afraid that the man made me scared of conflict.