This sounds like it may be a sign of "fistulous withers" which is just a chronic inflammation of this area. Many inflammatory disorders in all species can show up much worse on one day than it does on another. I would treat this with a combination of antibiotcs (just in case there is an infection in this area) and anti-inflammatory treatment. I would use phenylbutazone (Bute).
It would probably be good to have your vet look at him sooner instead of later....or at least give them a call and tell them what is going on. If this inflammation becomes too severe and too chronic, it can sometimes require surgery.
Meantime, my suggestion that you can do at home, is rest and water therapy. I would run cold water over the affected area once a day for 10-15 minutes. Sometimes this by itself can cool down an inflamed area.
Dr. Lyle
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