An idea....
If she sais "nothing" to begin with, then u have some communication problems.... but alas... lets just say that you ask her "whats wrong sweetie? U seem kinda down and not urself lately... wanna tell me about it?" and she said " no.. nothing is wrong" ... then u ask "u sure?" and she will say "YES! i'm sure" then in a joking manner u say " aaaare u suure?" She will probably catch on that u are saying it in a joking manner and probably smile and say something like"yes, i'm sure"... then u go on and say something like " hmmm... (her name), how long have we known eachother?" the she will answer... then u say " yes... and i think thats long enough to be able to tell when something is wrong with you" .. she will pobably say something like " NOTHING IS WRONG OK! sheesh!" ... then u say something like " FINE, then my name is BatMan!"
(if u havnt caught on, what you are doing in the above is getting her into a "joking" modd instead of a serious secretive mood)
At this point she will probably be laughing or in a "better" mood... then if she hasnt told u whats wrong by now u say something like " well... eventhough i STILL think that something is wrong, i'll respect your space if you dont feel like talking about it now... but know that I'll always be here to listen and help you with any problem that u have, and if i did something that hurt you, then i'm sorry"
Now you change the subject and talk about other things, and trust me, if she wants to say something, after saying all of the above, she WILL tell you without any more "convincing"
I said all of the above because thats how i handled a similar situation in the past, and it wored perfectly for me, so i thought i would share... who knows, maybe it will work for you too
Laterz...