But why was she so inept now?  Has she finally leveled out in smarts?  Is she now as correct as Ron (who is wrong ALL the time)?  My theory is no.  JKR couldn't have this female genius suddenly not know the answers. 

My theory involves Merope, who was Voldemort's mom.  I know, you're thinking, "Why would I even attach those two together?"  But Merope, whose man that she loved left her before she had her child, went into a depressed slump before dying and having Voldie.  Merope was a witch.  She could easily just use her magic to make herself better, which is what both Harry and Voldie had pondered.  But Dumbledore knew the answer; Merope was heartbroken, and felt no need to live.  She let go of her powers, even though she was perfectly capable of doing magic.

What does this have to do with Hermione?  Well, one:   Hermione suddenly can't do anything right.  This is a sudden thing.  Two:  Harry likes Ginny.  Another sudden thing.  Perhaps Hermione is feeling heartbroken, too.  Her love of her life is going out with her friend, and Hermione feels horrible, wronged, and just overall awful.  She starts snapping at people who she has never snapped at before (like Ginny, for instance).  And her powers are decreasing.