I have no insight about her "medical reasons". But, I don't think it was just a casual comment in her tweet; it was definitely premeditated, and she and her family had to know it would not go unnoticed and lead to more questions (possibly by the media).
What I do know is that since she already redshirted this season, she would have to sit out next season at her new school AND would lose one season of eligibility UNLESS she gets a waiver. So, I'm sure she and her family know that, and doubt she wants to go without playing competitive basketball two seasons in a row. There are no coaching changes, or "family issues", or "distance" issues that can be used as claims in her waiver request, so you have to come up with something if you want to be granted immediate eligibility.
Granted, she could always apply for a sixth year of eligibility down the road, but I doubt that is the mindset at the moment.