if you already had sex with the girl, that shows your doing something right. one of the worst fears some women have is when the man has sex with her, but afterwards, treats her differently (ie. talks less, tries to avoid her, leaves her for someone better).
your game never changes, only improves. your putting emphasis on her in order to judge your own actions and moves. if you act uncertain like you are now, she will be in control if you like it or not.
now to directly answer your question;
if she txts you asking you to call her sometime, do it. notice she said to call "sometime", that means whenever you become available.
you should take her out whenever you feel its a good time. in the future, twist the rule and tell her she will take you out(dont make this a habit, just something you should try sometimes).
if it makes you feel any better, pretend you never had sex with her. treat her the same as you would before you did her.