I looks like fluid retention and it's shown up on these here before. However, I don't think I've ever read what can cause this.
You're going to have to take it to a vet to correct this by draining and possibly have the fluids tested. Here is an example post similar to yours which might be of some help:
http://www.caudata.org/forum/f46-be...sick-axolotl-what-appears-fluid-build-up.html
I also included your picture in my reply so others can view it easier.