This tiny handful of words gives us a lot of information.
What we have here is the same process (being) written in two tenses—past and present.
The past tense version is written in a very specific way: he says had been, not was. This tells us that his nervousness led up to some event we don’t yet know about—making us wonder, what is it?
With the word am he clarifies and corrects his first statement, telling us he is still nervous now—which immediately makes us wonder, why?
So first these two words give us information, second they make us ask questions, and third—correcting himself in this way makes the narrator seem just a little compulsive, again raising the intensity and making us feel slightly uneasy about him.
