Ovarian Cyst or Pituitary Tumor?

Has anyone had positive pregnancy tests and it was not from pregnancy? I keep getting positive tests every week for a couple days and then they go away. It's been a couple months since my last period as I am nursing. I have been to the dr and got a negative blood test but it keeps happening!! I feel pregnancy symptoms off and on which prompt the testing. I think I'm dealing with something a little more serious. Any experience with this? My Dr won't do anything but give me another pregnancy test but by the appt it's gone again. I feel like I'm going crazy.