feeling like lovers; lovey-dovey mood
Describes the atmosphere or emotional state of being like a couple, often used for new or budding relationships.
They aren't dating yet, but they're enjoying the feeling of being lovers.
On the way home from the date, it felt just like we were a couple.
恋人 refers to an actual lover or boyfriend/girlfriend, while 恋人気分 describes the mood or feeling of being like lovers, not necessarily an established relationship.
気分 means mood or feeling in general; 恋人気分 specifically refers to the romantic, couple-like mood.
Compound of 恋人 (lover) and 気分 (feeling, mood). The exact origin is uncertain, but it likely emerged as a natural expression for the romantic atmosphere between two people.