I have read about floating Axies. This is mostly due to bacterial infection.Considering the temperature you quoted, and the fluctuations that would have happened at night, I find it hard to believe it would have been a pebble - but who can say for sure?
28 degrees was WAY too high (24 is absolute max, 18 is better) and needed to be addressed somehow. A fan blowing over the tank may have helped a little, but who can say. High temps stress them out AND promote bacterial growth - a deadly combination.
Frequent water changes could have helped as well as salt baths, but it would have been easier just to reduce the temperature.
It is even possible to massage them in emergency, but make sure you use well washed latex gloves!
Too late now of course, and I am sorry for your loss.
From: http://axolotl.org/faq.htm
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Often, some stress will lead to a small bacterial infection in the gut. This would lead to a gas build up, and the floating. It is possible that the axolotl swallowed a large amount of air into its digestive system too. Warm temperatures (over 20 °C / 68 °F) don't help. Anything over 25 °C / 77 °F is far too warm for axolotls - just in case you didn't know.
As to how to solve the problem, axolotls find it stressful when they can't touch the bottom. I suggest you lower the water level so that the axolotl is touching the bottom, but still submerged. The fact that it can touch the bottom should help to relieve the stress and hence help it to recover. If you keep it at temperatures over 20 °C / 68 °F it will probably make it harder for the axolotl's gut to keep up with any bacterial activity, so try and keep it cooler. As long as it eats, that's a good sign. Sometimes it is possible to massage the animal's abdomen to help it pass the gas, but it's hard to do without hurting and/or stressing the animal. It's safer to do what I've suggested and let the animal pass the gas on its own.