There is a common rule in conversations, most women know it and use it well. Men,not so much.
"If you don't like what's being said, change the conversation."
You'd do well to remember that one. For now, and the future.
Her action of changing the topic meant that she wasn't very open or comfortable to your suggestion of " we should go for drinks or lunch".
It could be due to a number of reasons. Maybe there's not enough of a comfort level between you both for her to agree to meeting you alone. Maybe there's not enough attraction on her side to be able to want to see you, as soon as you suggest a meeting. Maybe, maybe.
I would suggest you to disregard this and continue the conversation as if this never happened. Try to flirt with her, try to have long, engaging talks with her. As much as you can. You will have to be patient with it. But, if you strive at it continuously, it will pay off and you'll get your date.
ok the next time i text her should i ask her out again?