sour lump
Someone who is acting uptight or irritable, thereby ruining everyone else's good time.

Emma: "They're just a sour lump in the mix,"
Angelica: "Don't be such a sour lump, Liam!"
Liam the Sour Lump: "That wasn't funny, guys. >:("
Friend 1: "What's the matter with him?"
Friend 2: "Oh, don't mind him. He's just being a sour lump like always."