What is going on with Thirteen? I haven't watched a few episodes of last season so I have no idea...

Thirteen suddenly left an envelope with her demand for a leave of absence on House's desk in "Help me" (season 6's finale). In the first episode of the 7th season Foreman tore that letter and found out that she had a plane ticket to Rome. She was really mad because of it. He assumed that she was going to a research institute for Huntington's disease from which she is suffering. She let everybody believe that story, got lost the following day and Foreman called the institute only to find out that she was lying all the time and they had never heard of her in Rome. So, basically, nobody knows where or why Thirteen is gone. Not even House. (Hope this helps! Excuse my English!)