Called my obgyn's office and talked to a nurse. She said to wait until Friday, take a test at home…

Called my obgyn's office and talked to a nurse. She said to wait until Friday, take a test at home and if it is still negative then go in for a blood test. I am hoping it is just stress but I'm not having any period symptoms except bloating. My back usually kills me and cramps are so bad they might as well be contractions but none of that is going on. Just slight back aches, bloating, fatigue, and cravings. But I've gotten two negative tests. If I was pregnant how early could I be to not have a period but negative tests?