Q: Syncing/Authorizing Error

I am trying to sync my daughter's iPhone, which I have been able to do successfully in the past, but now my iTunes is telling me it cannot sync because my computer (the only one we have) is no longer authorized.  So I go through the process to authorize, it says it is already authorized, then I try to sync again - and I get the same error.  I'm stuck in a continuous, frustrating loop!  I make all purchases under my iTunes account and Apple ID, and the teenager says she hasn't made any purchases under another ID - does anyone have any idea what my issue may be and how we can fix it?  I'm ready to throw her phone and my Macbook out the window!