Clearly you get what I'm talking about and have had similar experiences. It happens around other activities also, not just firearms, and I've has such great experiences in many. I got the same feeling when I was playing American Football.
I think that .22 - o/20ga sounds like a decent firearm to start with and I can imagine how it would have been for you shooting slugs for the first time. I used to use a 12ga under and over for my back up when wild pig hunting and used it a few times...brutal but fun.
Imagine, as a kid, always shooting the .22?
Then, suddenly, the 20 ga.!!!
From that day forward, my shoulder completely understood when my dad was telling me, "seat the butt firmly"
One must teach one's shoulder a lesson with a shotgun at some point, earlier the better. Getting it wrong only happens once, usually.
So very true!