Your rabbit is more than likely eating cecotropes which are small round moist pellets from the cecum. The cecum is a part of the rabbit's digestive tract where the indigestible fiber is broken down to make it into digestible nutrients. Once it is broken down in to those digestible nutrients it is passed through the digestive tract in the form of the small, round moist pellets called cecotropes. They are passed out of the rabbit & then redigested when the rabbit eats them. It is a natural part of a rabbit's digestion process. So you shouldn't be concerned.