This is the 19th most frequent Japanese word.
なっ
"なっ" is not a standalone word but could be a contracted or colloquial form, often used in speech. It may derive from "なっちゃ" (short for なってしまう), meaning "ends up being" or "becomes." Context is needed for precise meaning.
In this sentence, 'なっ' is the stem of the verb 'なる' (to become), combined with an auxiliary 'ちゃった', indicating that the action of becoming summer has been completed.
夏になっちゃった。
It has become summer.
Here, 'なっ' starts the conjugation of the verb 'なる' (to become) in reference to the state of feeling better.
気分が良くなった。
I feel better.
The 'なっ' represents a conjugation of the verb 'なる' (to become) into an intermediate form, leading to 'なって' to indicate progression or transformation.