Does he wear a breast collar or something that may have rubbed?
Could he possibly have been kicked by a buddy?
One time my boy came in from pasture with a squishy, fluidy spot here on his chest. It didn't look too horrible, but he would flinch if I brushed it too hard. The next day or so, it had kind of firmed up and sounds like what you described. It slowly shrunk/healed and was gone in a week or so. I assume his pasture mate had gotten him with a hoof.
Did you notice anything on Dez's chest yesterday?
Is it really the size of a fist? Or could you have possibly missed it in the past since he wasn't shaved?
Maybe it's just a weird man bump type thing.