At her reaction he moved his mouth back up to the small nub sucking on it once more as he brought one hand down and teased her entrance with one finger.  He would prepare her for him this way so it would not hurt as badly later when he entered her for the first time.  he pushed his finger in slowly to his first knuckle and stopped everything but his assault on her with his mouth making sure her body was ready for him to push in further before he did.