Mine did exactly the same thing. It seems like an adjustment period they go through when you move them inside. Last year, I brought two inside. One lived through the winter and one did not. I put the one that did back outside in the Summer and it was fine, happy, and bloomed.

I'm not sure what to suggest to make it better.

I found this today and it seems helpful. It even talks about how they will lose leaves.

http://homeguides.sfgate.com/overwinter-hibiscus-tree-21829.html

Update on the hibiscus migration to the indoors. The one whose leaves turned yellow appears to be bouncing back now. It has some new growth shoots on many of the stalks.

How is yours doing now?