Ouch. Ask to see proof thats she's posted it. If she does not have proof then I would say ask her to return your item back and you no longer wish to do the trade.
Playing the 'someone else posted it so I dont know' game is sending off warning signals everywhere for me.
If its been two weeks past when it was supposed to arrive I think you should request she send it back to you right away, WITH recorded delivery and proof of postage.