You can use exterior paint on interior surfaces. There are important differences between interior and exterior paint.
Exterior paint has added chemicals that help with UV protection and inhibit mold growth. Plus, exterior paint is more flexible (rubbery) and takes longer to cure. Usually remains tacky to the touch longer when compared to interior paint.
Also, it doesn't work the same for interior painting, but this won't affect you so much. These are different types of paints. But, for your bathroom walls you probably won't notice too much of a difference.