my last real period was from the aug 15th 2012 to aug 19th 2012 i have a regular 28 day cycle and me and my bf had unprotected sex on sept 4th on sept 16th one day after my period was due i had light spotting lasting hours everyday from sept 16 to sept 20th i took 3 pregnancy test up to sept 21st and all were negative. Can i be pregnant? and when can i test again?