Not Just Basic Biology

Rainbow Dash's been feeling strange lately - more than usual. Creeping thoughts invade her brain whenever she isn't actively trying to keep them away. It's so much easier to ignore the problem than figure out what's really gone. That is, until a new pony visits Ponyville. A new pony with shocking new information to share with Dash. This new pony, and the little things quickly stacking up that Dash's been having a harder and harder time ignoring, leaves her with a realization that changes everything.