A sunken stomach usually means internal parasites which are very common for DPs. I had couple batches of DPs all of them all had internal parasites when I got them and lived for a little over a year.
As was mentioned they are small but need a lot of room and a LOT OF PLANTS if they dont have enough plants they get stressed and exhibit symptoms similar to yours.
I used to visit this site a lot good info
http://www.dwarfpuffers.com/
Here is a pic of my old setup
Good Luck to you they are definately my favorite freshwater fish.
Ken