Girls/women are funny.
Eyeball you all day, but when you step up - she's thrown off by you actually talking to her.
Embarrassed for you. (You thought I was feeling you);.
Embarrassed at the situation. (We're at church! You could be at Church's Chicken and get the same response);
And low-key embarrassed at herself, for being so obvious. (Which is why she might keep staring).
Speaking to the audience/ lurkers
The real lesson is that trying to nail down what exactly went "wrong" is a fools errand.
You won't know, because she doesn't know.
They don't know why they do what they do.
She might give you a reason, but whatever she says is really only her trying to reconstruct the scenario in her mind, and then give you a plausible explanation, so that she can get out of the conversation and still keep her ego intact and keep yours down.
Stop looking for these things.
Because if I can hit you with 5-20 plausible reasons why one particular broad might do something there are infinite number of other reasons.
All that said - these broads change their minds at the drop of a dime, with zero new information.
Chick will have a change of heart, and in weeks to come she might listen in in your conversation, find a way to be in the same space as you, be in your line of sight, talk to the last person you just talked to... All types of creepy behavior. (when men do these things, it's labeled creepy).
This creep before is her putting you in a position to try again.
It's happened to me, and I've seen it happen to others. Approach a grown woman respectfully and discretely, especially one giving choosing signals, only to have her dismiss dude. Few weeks later, she realizes all of those initial feelings were not her real feelings.
Even though y'all could have been in matching tshirts, she can't approach you....
Lemme be clear, She might strike up a conversation, but she won't make herself vulnerable to hearing you say no to her. She has to preserve her sense of self.
That's the game she's playing, what many play.
Playing games is not having game.