very faint line on pregnancy test!

Hi there I toke a pregnancy about 3 days ago and it was a very very faint positive soo i went to the doctors today and showed my doctor and she said its negative i told her it was very faint and she said doesnt matter its still negative! Im very confused on what too believe and I have the 5 year coil aswell and scared that if i am pregnancy i dont want the coil harm the baby or myself! Somebody please help?? What should i do?.