What happened to Desdemona in "Middlesex"? Why did she take to her bed for 10 years after Lefty died?

Desdemona is an interesting character who is deeply superstitious and riddled by guilt for having married her brother, Lefty.  She also is almost hilariously dramatic when things go wrong, as evidenced by her habit of fanning herself with "six atrocity fans" when agitated.  Throughout her life, Desdemona tries to stop the bad luck she believes she inflicted upon her family through her sin of incest, even to the point of having herself sterilized.  She withdraws from her husband and, eventually, from the world.  When she takes to her bed after Lefty dies, there is nothing wrong with her physically, so her action is essentially hysterical.  Cal describes the situation perfectly, calling her grandmother "a sick person imprisoned in a healthy body".