My first impression, especially with the quick onset and the continuation towards ulceration, is that this is a severe burn. Any possibility of this? Beyond getting to close to a heating element, it could also be a chemical burn; sprayed with the wrong bottle, kids fooling around, etc.

Either way, I think this is beyond the scope of internet help and I would recommend a vet visit rather soon. The ulceration is an open window for bacteria to enter the body, so in addition to the veterinary advice, I would set this frog up in a simple, sterile type enclosure (that still meets it's environmental needs).

I hope this helps, please keep us updated!

Ian