It's not unheard of... isn't always unhealthy or a warning sign. During this ast holiday season, I was already riding a little high on my weight... the pool closed down for repairs, I was eating too much, eating the wrong things, and imbibing too much as well.
Short of it, come January 1, the lady and I finished our holiday indulgences, and we began Dry January on the 2nd. With that, I also started eating right again.
I went from a bloated and short-term weight of 260 on December 31 to about a 10 Lb drop in just over a week of proper eating, exercising, and abstaining from alcohol. Yesterday I weighed in at 235.
Still dry, too.
To be fair, I swim a minimum of 1 mile, 5 days a week when the pool is open. Sometimes as much as 2 miles in a single session. I have been actively working on my fitness for several years; that I had crept up to 260 again was an unfortunate confluence of several things (some my choices and behavior).
Of that initial weight loss, 5# in 3 days was largely just Bloat and Inflamation going away as I got my system to clear itself out.