Pretty much the same story here. I went from winning local tournaments and consistently beating everyone I knew...to stepping into the world of nationally ranked players, and losing horribly. Kind of a funny story when I made that switch. Guy casually mentioned to my employer at a business meeting that he played chess. So she got all excited and asked him to play me, and made some sort of bet to settle some thing that had been in dispute at the meeting. We agreed, I was all internally triumphant, thinking "haha! I'm going to play this off casually then slaughter him yay!"
After about five moves in he starts to get this confused look on his face that just kept growing as the game continued. Eventually I make a move, and he just...stops.
Looks at the board, leans in frowns, then looks bewildered. Eventually he shrugs and basically says "ok, I don't get it. I don't see it. You're probably going to win, but I'm sorry...I just really don't see it."
So he makes this totally innocuous move that to me looks like it has no bearing on anything.
I was in checkmate, like 10 moves later.
Three moves? Absolutely. Anyone can do three. Five? Sure, I've done five. Not consistently, but I've done it. No, this was genuinely like ten moves ahead that he saw checkmate. I asked him to walk me through it, and he did. I remember asking him if he was a grandmaster and his response was "no, no...nothing like that."
Pause "But I've played a few."
That was about when I realized I was a big fish relative only to the size of a very small pond.