I'm sorry to hear about your frog, and that i didn't see this earlier -- sounds like Bloat /Dropsy -- i lost one of my frogs to it a few years back. Its the same thing that can make your fish look like pinecones (so bloated the scales stick out).

I'm sorry its too late, but just in case it starts to happen to your other little guy, heres a couple links that might help for treatment --

http://www.fishpondinfo.com/frog4.htm

http://allaboutfrogs.org/info/mypets/dwarfs.html