How does the narrator feel immediately after he commits the murder? do his feelings change? if so, how and why in tell-tale heart

Immediately after he commits the murder, the narrator feels very calm and confident, he describes the whole situation in which he disarmed the body:

First I cut off the  head, then the arms and the legs. I  was careful not to let a single drop  of blood fall on the floor. I pulled  up three of the boards that formed  the floor, and put the pieces of the  body there. Then I put the boards  down again, carefully, so carefully  that no human eye could see that  they had been moved.

Then, while he is talking to the officers, he starts feeling guilty, so guilty that he imagines the sound of the heart beating. He thinks that the officers can also hear the sound and that they are setting a trap. He ends up confessing the murder:

No! They heard! I was certain of it. They knew! Now it was they  who were playing a game with me. I was suffering more than I could  bear, from their smiles, and from that sound. Louder, louder, louder!  Suddenly I could bear it no longer. I pointed at the boards and cried,  “Yes! Yes, I killed him. Pull up the boards and you shall see! I killed  him. But why does his heart not stop beating?! Why does it not stop!?

. a function is a relation in which each input has only one output. in the relation , y is a function of x, because for each input x (1, 2, 3, or 0), there is only one output y. x is not a function of y, because the input y = 3 has multiple outputs: x = 1 and x = 2.