Sometimes people want that which they don't or can't have. Then, once they have it...
I suspect that she's playing a game and was testing your reaction to see if the old sparks can be reignited. But, that sends a mixed signal given that she had only recently dumped you. Maybe, she dumped you for someone else and it's not working out; so, now she feeling something like buyer's remorse. Anyway, I wouldn't trust her, just yet. Oh, I also think that, under the circumstances, your reaction was quite correct.
Maybe she realized after she broke it off that she is unhappy without you and wants you back.
Only she knows the answer to this question. Talk to her and find out.
Talk with her and see what is going on, see why she ended things and why she wants you again.