Answer: Episode title!
In "The Isolation Permutation," Amy was devastated when Penny and Bernadette went wedding-dress shopping without her. Heartbroken and lonely, Amy wallowed in her sadness by playing depressing tunes on her harp.
Thankfully, by the end of the episode, everything worked out for Amy, who snagged an invitation to look at dresses and a great pity-snuggle session out of Sheldon! Who says being blue is a bad thing?