Reproduction, it's a perfectly valid thing that makes total sense. When a mare and stallion love each other very much... Blah... Blah... Blah. Here's a foal. That's how it's supposed to work. Except for when it does not.
Heat season was always the worst for Twilight, especially since coming to Ponyville. However, thanks to her accident with the Want It, Need It spell and her clones, she has a great outlet this season. The consequences, however, are big.