That's weird. But I hope he's all right! Definitely keep a close eye on him. If he's sick, I'd also guess it's some problem with the bladder. In any case, keep your water very clean if you ever suspect any of your fish is sick. I'd recommend lowering nitrate below 10
PPM when fish are sick. Increasing the temperature to around 82F would be good as well.
In my 10 gallon tank, when I still had Neon Tetras in it, my water was always excellent - nitrate was never more than 5 ppm. The tank was very mature (running for about 1.5 years since the last cycle). And yet, one of my Neons got a horrible case of Dropsy out of nowhere. It died all swollen, looking literally like some puffer fish. My point is: sometimes you'll get sick fish even in the healthiest of tanks.