This expression came from the H-game (PC)/PS2/DC/anime Kanon. Ayu Tsukimiya's line (and probably the most famous throughout Kanon). This is her expression when she moans, or whines (like eeeehh, but more original).
Darn though. She had a sad story. One of my few favorites besides Nayuki.
Ayu: I'm cooking cookies.
Yuuichi: I bet they'll taste terrible.
Ayu: Uguu! I've learned how to bake from Akiko-san!
Yuuichi: You can't learn how to bake with just watching in a day..
..although I'll eat them if you manage to make them look good.
Ayu: Really?! Then I'll do my best!
When a person is constipated.