There's a HB that I've known for a while. Back when I was in another relationship she would IOI me every now and then, but I never really bothered / responded. Now when she found out I broke up with my ex she was giving me really strong IOIs ("so i heard you're single again - so what are you gonna do about it?

", "you know, you and i should get drunk 2gether sometime, you know JUST the two of us", later that night she was also complementing me about me smile). Sadly, I didn't exploit that situation but instead ignored her approaches quite bluntly (I guess I was a little overwhelmed + I naively thought I could easily get with her some time later after looking for other chicks).
Anyway, after some time had passed i thought i could subtly get on a date with her. So I casually stopped at her work two times to "stop by" while being on the way for something else. Although she was really happy that I came by, she blocked all my attempts to establish a flirt - very inconsistent with her previous behaviour, so i didnt find the right hooking point to ask her on a date. I wanna give it another shot, though.
My question to you guys:
What's the reason for her change in attitude? Is it revenge for that I had rejected her at first? Does she want me to be more direct/aggressive? Or did she simply lose interest?
She's gone to her hometown now for vacation. My plan is, after she comes back, to ask her to come to a bar to meet with some friends (who will all "coincentally" have cancelled last minute, which will leave only the two of us). While she's still away I thought about texting her something casual that will reinforce her good feelings for me. Any ideas for something short and smart?