What zone are you in? If your in a cooler USDA zone it could be that it's just going dormant. How often are you watering? Are you fertilizing at al It could be that the leaves are burnt from too much fertilizer. Hope that this helps. I know other growers will be here to help you. I wish the best of luck to you and your plumeria:)

The plant has been in the sun, how often should it be watered. I did fertilize it last week.

A few more ideas:

1) spider mites

2) needs potting to larger pot

3) dried out too much.

I'm thinking spider mites. Hose off the leaves top and bottom and then treat with neem oil or a 3 in 1 product.