Can you check the soil moisture around the base of the main trunk, notplantsavvy? It could be that the soil around the root ball has dried out. In cases of soil moisture issues like that, the leaves begin to dry out from the edges and brown out completely. It happened to me two winters ago (shrub was planted in the ground though) due to a sprinkler system that was left turned off too long in the middle of a dry winter. Make sure that the area around the main trunk receives sufficient moisture.

On the other hand, too much water can also cause similar symptoms specially if the soil does not drain well, if the pot has insufficient drainage holes or if the pot is allowed to sit in water. In this case, the roots slowly develop a fungal infections called root rot and are unable to provide the plant above the ground with the amount of moisture that the shrub needs. The plant browns out just as if the plant was not receiving enough water because, in a way, it was not. If you examine the roots of these plants, you will notice that the roots are brown.

I assume that you have no pest issues with this camellia because you reported no damage to the top/bottom of leaves, no damage (cuts, etc) to the trunk and things like that.

Luis