This is normal.
Roosters will favor a hen for a while then move on. You probably have a cute Rhode Island Red hen, in the eyes of the rooster. The hen will be just fine for the most part, but look for problems. If the action of the rooster is such that it draws blood, the rest of the chickens will be strongly attracted to the blood, pecking at it relentlessly. This can cause injury, infection and death. If you see blood on your hen, immediately segregate her from all your birds until she heals. If your concern warrants action, you can segregate her any time to give her a rest. Her feathers will grow back in time.
There is a neat little product on the market called a Hen Apron. It fits over the back of the hen to prevent injury.
See: the Hen Saver
These offer protection and allow the hen to remain in the general population. Cost $8-12 plus shipping.
The ratio of roosters to hens may be something to consider. A couple of roosters will keep a dozen hens in the buff. Even with 1 rooster for 20 hens, because they have favorites, you can end up with bald hens. Rather than segregate a hen, it may be more practical to segregate the rooster, particularly in the spring when they tend to get overzealous.