She Flirted at Our Party, Now I'm Torn

I (28m) caught my girlfriend (27f) flirting with a coworker at our housewarming party. She's always been friendly, but this felt different. I confronted her, and she insists it was harmless fun. Now, I'm questioning our relationship and don't know if I'm overreacting. I want to trust her, but part of me can't shake the feeling that something's off. Any advice? #WhatWouldYouDo #TrustIssues #RelationshipDilemma