IKAIKA13
First of all I'd like to say that is the biggest so called 'Pink Pudica' I have ever seen.
It could be your plant is going dormant - where do you live? Here in South Florida all the plumeria are starting to lose their leaves and entering their dormancy, from which they will emerge in the early spring.
If that is not the case, I would indeed think about overwatering being the cause. I assume you have checked for insect infestation?
