Sorry man. Rejection hurts. No denying that. But whatever right? On to the next one
BUT
If she actually said that, she was not worth your time. Sounds very rude, and borderline Abuses will lead to bany (no offense) And don't ask the girl "if" you can sit down, just sit down. It is no big deal if you play it off like it's casual. Say "Hey" while your standing, and just grab a seat in mid-sentece. If you do it right, it's pretty smooth. Provided she was sitting alone of course. That another thing, approach when the chick is alone.
I don't claim to be some expert. But this is an extremely common method employed by girls when flirting. It all depends on the "type" of eye-contact. If it's a soft DIRECT look into your eye, and you do the same, and your INSTINCT (Trust your instincts bro) tells you something, I would say yes, she is attracted to you. Does she smile though? Looks away quickly? or just stares?