ニヤついちゃう
niya tsui chau

Godan Verb (ku)
Godan Verb (u)

The completive aspect (contracted form) - indicates completion or regret of ニヤつく

1. Smiling to oneself (often with a mischievous or self-satisfied air); Grinning (often implying trouble or smugness)

Completed Action
Colloquial