Could it be a slow onset of root rot or some other root problem? As a surface rooter, they can be tricky in pots if the soil stays too wet. I let mine get dry down a ways in the pot before watering again (been there done that with too many acid-loving shrubs). Often the symptoms are a sudden wilt or drooping of the leaves that doesn't go away when the plant is watered.

I lost both my Camellias (Korean Fire, Winter's Star) to root rot this year, probably from having 4 feet of snow that melted slowly. The Camellias NEVER had that much water in winter before. Lessons learned: 1) Drill MORE and big (1/2") holes in the bottoms of the cedar planters I use to hold Camellias 2) Make sure the containers don't touch the ground, but are elevated so that water can pour out the holes I just drilled. (I now use small landscape pavers as cheap pot feet.)

Before I figured out what was going on, I replaced the dead/dying Camellias with Japanese Maples, which seem to do better overall in containers. Now I have an excuse to find NEW Camellias to put in the old containers, which are currently baking in a sunny, unused side yard (no water) to hopefully kill any root rot organisms lurking in them. If I can get some copper sulfate or potassium permanganate, I'll paint the insides of the containers and let that dry to make darn sure, then rinse out all the chemicals before planting new Camellias.