。
I feel especially good.
Literal
I [topic-は] especially feeling [subject-が] good.
特別 ('special, especially') is a な-adjective/noun used adverbially here (without に, which is also grammatical). 気持ちがいい ('feel good, pleasant sensation,' literally 'feeling is good') is a set phrase — yet another example of the double-subject construction in which a large topic has a small subject predicated of it.