0 blunders, 0 mistakes....and yet I lost to a removed ELO player

Here is the game.
https://www.chess.com/game/daily/ removed

I played decently well. The engine rated me as playing at a 1550 rating, which seems fairly accurate. I've been rated as high as 1700 here.

In a game of this length, making no mistakes should be enough to crush even a 1200 player in my experience. And yet my opponent, rated under 700, played at an 1800 level according to the engine with two brilliant moves - including a queen sacrifice that would have lead to mate in 2.

I'm not prepared to say that my opponent was cheating necessarily, although maybe. Nothing seemed inhuman about their play. But it was laughably above what you would expect from someone with their rating.

Any theories as to what was going on here?

And sure, I'll take any constructive criticism on my game play. This was a very claustrophobic game for me, my pieces were stuck and white's pieces were all working together. I thought white had weakened the pawn structure protecting the king, but couldn't figure out how to exploit that.

You can't post a game and ask for opinions on whether or not someone was cheating.

All you should do is to report them.