I just had this exact thing happen to me with my dwarf african clawed. I am sorry for your loss.

Edit:

I am also going to have to lean towards the bacterial aspect of this. Mine did the same thing with the floating and not diving thing but it also had the red patches, the lethargy, and the loss of appetite. Mine also died while floating rather than sinking.

I would go and have your other one checked out in fear that he has it as well. I lost one frog when I first got them (in december) and just thought he died naturally never once thought it could have been bacterial from when I got him. I didn't know enough. And a couple months later here I am with another dead dwarf african clawed. I don't know if it's directly related but there's always that possibility..so to be on the safe side i'd run the little fella to a vet.