I've noticed a pattern with myself and I'd like to know if someone else can relate. I have a score which fluctuates between 1200-1300 on chess.com and I've noticed that after a great day of defeating opponents (some even higher rated than me by as much as 200) I tend to play inaccurately for the next couple or days or so. It's like some days I'm really playing good and even giving the computer a hard time and other days I just mess everything from openings to mid-games. Is this common? I play a lot of online chess regularly and have noticed this fluctuation from time to time.
At least for some players, yes, although how many games, the type of games, and in what period of time, causes burn out, I would assume varies from player to player. I've been reading "Bent Larsen's Best Games," and he mentions how he was drained after playing several consecutive events, but after a month's vacation, came back refreshed.
The fluctuation within a range but never breaking out of the range means you have hit a ceiling and are flatlining. Think of it like the stock martet whereby you are consolidating, and take this opportunity to study and change your way of thinking about the opening, middle-game, and endgame. Get outside your comfort zone. read a difficult chess book that speaks to a full class above your current skill, etc., etc., and apply what you learn.
Note: you'll probably get worse short-term doing this, but hang in there.