Sorry to hear that Lunchbox. It's hard to say whether or not the fat belly was related to it's death or not, without doing an autopsy. From the pics I don't see any redness that would indicate an infection, and it sure looked like a gravid female to me. I wonder if internal parasites may have been the cause. In any case, it's not uncommon for otos to die within the first few weeks. I've had pretty good luck with my stock, but it seems like it's usually a gamble.
Again, sorry to hear about it - I was really hoping you'd end up with some fry!