Please consider writing this woman a letter -- just what you have put in your posts here.

When I have not wanted to engage with someone, I have found it helpful to say something like, "I'm not going to be able to ..." (meet, chat with you right now...etc). If they don't get that as a 'no' and they ask, "Why?" I look them in the eye and say, "Because I am not feeling well right now."

No need to elaborate on that. If they are totally clueless and continue to ask why then I leave/walk away.