Adult Discus sometimes may suddenly stop eating for a few days at a time. This is not extremely common, but does happen. If you suspect it's a bacterial infection, don't raise the temperature. Observe the fish for a day or so and look for further symptoms or improvement. If only one or two fish are affected in a tank of many, you may wish to move them to your hospital tank, in order to prevent possible infection of others.
Medication is also more inexpensive and easier to administer in a smaller tank.
PH shock is a very common problem. Even though you think you changed the water and did things right, this is a common final diagnosis for unknown stress on Discus. It's very easy to get in a hurry and push your PH adjustment quickly with water changes, especially if you don't use pre-treated, stored water. It may seem dumb, but it is easy to make simple mistakes like adding some chlorinated water to the tanks, too. Be careful with this, as well.