Caution: there be swearing.

When she was young, Tonya's birth father was physically abusive. Her response was to evolve early so she could toughen her skin to the blows. When she finally evolved into a butterfree, her folly was soon realized. Her wings would never be able to sustain her for flight. On top of that, it occasionally gives her pain.

She has to visit the doctor regularly to make sure it doesn't get any worse. This is one of those visits.