In the Laghubhagwatamritam, the following description is there. "Sankarsan of the second quadruforms appears as Rama taking with Him Sesha Who bears the global spheres. There are two features of Shesha. One is the bearer of the globes and the other is bedstead servitor. The 'Sesha' bearer of the globes is the potential incarnation of Samkarsan and such He is sometimes described as 'Samkarsana' also. The Bedstead feature of Sesha always presents Himself as the eternal servitor of the Lord."
This Ananta Sesha Personality of Godhead glorifies the potencies of Lord Krishna constantly and although He sings like that, there is no limit of His (Shri Krishna's) potencies. The Kumaras (Sanak, Nanatan, Sananda, Sanatkumar) hears Srimad Bhagwatam from His lips. He is always full in ecstasy by glorifying the Lord.
So far His servitorship feature is concerned, He transfers Himself in the form of spiritual canopy, slippers, bedstead, pillow, garments, rest, residence, sacred thread, throne, etc. He serves the Lord in so many forms of services. Practically He only knows what is actually Krishna and therefore He is known as 'Sesha.'
Lord Nityananda is the sum total of so many plenary expansions of Lord Shri Krishna. We can simply make a rough estimate of Shri Nityananda Prabhu simply by delineating on these subjects.
In the highest conception of the Absolute Truth, there is no distinction between the incarnation and the incarnator. The original eternal Form of the Absolute Personality of Godhead is called the incarnator whereas His plenary expansions in all subsequent stages are called incarnations. All of them are one and identical. As such some devotees who have no specific knowledge in the matter of different incarnations, do sometimes designate Nityananda Prabhu as the incarnation of Ananta just like it is sometimes assumed by some such devotees that Lord Krishna is the incarnation of Vamandeva. As such statement are made by pure devotees, they are as much right in the ultimate conception of the Absolute Truth.
This is also discussed in the Laghubhagwatamritam as follows:—In the slokas of the same the author author has refuted all arguments in the conclusions of "Krishna is the incarnation of Vishnu in the ocean of milk" "Krishna is the incarnation of Vasudeva the first plenary expansion of Narayana in Vaikuntha." "Krishna is the pastimeous Form of Narayana" etc. The author of Laghu Bhagawatamritam quotes a sloka from the Bhagwat 3.2.15 which states "Lord Krishna descends from His abode in this material world, accompanied in His own Person, by all His expansions as Narayana in Vaikuntha and all other plenary portions dominating over the material and spiritual worlds—exactly in the manner as fire comes out of match wood manipulation, when His peaceful devotees like Vasudeva, are disturbed by some conditioned souls like the awful Kamsa or similar other demons."
Therefore Lord Krishna descends merging with the identities of Narayana in Vaikuntha. He is always in link with all the Purushavatars as also other incarnations like Sri Rama, Nrisingha, Varaha, Vaman, Naranarayan, Hayagriva, Ajit etc. At Vrindaban sometime Lord Krishna exhibits the functions of such incarnations. Therefore the statement of Brahmanda Puranam that Lord Shri Krishna appearing as the son of Nanda Maharaj is no other than Purushottam or the quadruforms in Vaikuntha or the Lord of Setadwipa, or Narayana the friend of all Naras (living beings). All different incarnations do merge into the Person of Lord Krishna like the burning sparks of fire which come out of the fire and again merge into it."
On all these authoritative statements it is concluded that Krishna's being is Naranarayana, Upendra or Vaman the Vishnu lying over the ocean of milk or the Lord of Vaikuntha etc. as defined in different Puranas. And as such there is no discrepancy in such statements of mixing up between the incarnations and incarnator.
Now coming to the original point that Shri Chaitanya Mahaprabhu is Lord Krishna Himself, it is natural to conclude that Lord Shri Chaitanya Mahaprabhu could perform all the different parts of different incarnations. And everything was due to Srila Nityananda Prabhu who serves Lord Chaitanya in different manners. Sometimes He serves Him as the Preceptor, sometimes as a friend and sometimes as a servant. And all these parts were displayed at Braja during the advent of Lord Krishna. The two brothers in a childish way, played the part of oxen and fought with one another in a mock-fight. Sometimes Shri Krishna massaged the legs of Valadeva. Shrila Nityananda Prabhu always represented Himself as the servitor of Lord Chaitanya. In the Srimad Bhagwat the following statements are made in connection with mutual playful dealings between Krishna and Valaram. It is said there that (Bhag 10.11.21) the two brothers like ordinary boys imitated the groaning of oxen fight with one another; and sometimes they imitated the natural voices of ducks and peacocks Both of them wandered in the wilderness by such imitation of animal voices.
Sometimes, tired by such playful pastimes Shri Krishna Himself lay down on the ground resting the head on the thighs of the cowboys and some times He caused Valadeva to lie down and massage His legs with His own hands.