闹心 is a stative verb (s.v.) describing a state of inner turmoil — you feel annoyed, bothered, or queasy, often because of something that won't stop nagging at you. It's more colloquial and visceral than 烦恼 (which is more general 'worry') — 闹心 implies something is actively 'making a racket' in your heart/mind. You can use it for emotional annoyance (a rude customer) or physical queasiness (motion sickness).