What Causes Delayed Periods With Light Spotting And Stabbing Pain Under Rib Cage?

Hi there my period is 12 weeks late and my gp has confirmed pregnancy in the surgery I had light bleeding last night and today so I went to the emergency room and they have said I am not pregnant how is it possible to have a false positive if it has been confirmed by my gp I also have a very sharp stabbing pain under my left rib cage and in my back right now on really worried can you give me any information on what might be causing this thank you