Sweetie Belle doesn't know what's going on with herself, and goes to her sister for help. Extreme awkwardness ensues. Don't read if you don't like "if ponies had periods" stories.
Diehard Brony/Whovian. Discord is best villain, Fluttershy is best canon pony, and askdiscordwhooves is best pony (check him out on tumblr). any questions? Hope to be a valuable member here!