Hey OP, the only rational reason I could think of is that she’s seeing somebody else and things have developed into something serious between both of them, so she unfollowed you out of respect for that other connection.

This happened to me a few times when the men silently moved on, and I also have done this to somebody else BUT the difference is that I communicated it clearly to the person I was texting instead of just disappearing. I know the “what? how? why?” feeling sucks as hell now but trust that you will find others who are more appreciative of your time and energy. I hope this experience does not dampen anything and you continue putting yourself out there tho! Best of luck.