It is quite normal to happen to aquatic newts. Highly oxygenated water from a new water change can trapped between the linings on the newt's skin. Thus, it started floating. After a while, your newt should be recovered from it. A newt with dry skin entering water will have this issue as well. I have also heard about some harmless bacterias causing gas to form in the newts, which I think is temporary. Anyhow, I have never encounter the 3rd case. Sometime if a newt is not filled up with food, they might float as well, from my observation.
I guess, with more people sharing some comments, you can get a better picture. For now, I can tell you that this should be normal and no need to worry. I have observed this many times in my Cynop Orientalis.
