Pregnant :)

Well to my complete surprise, shock and delight I have found out I am expecting !! Now by my calculations I am about 8 weeks 6 days from my predicted conception date, but from my lmp I'm considered 4 weeks! However this is impossible as my DH and I decided not to try last month and continue in the new year!  My "period" was late last month but only by hours! I usually am extreemly regular and come on at the same time    Of day every time! The period was more like a mucus with blood and eventually turning into a bleed a few days later! I am so confused as we both know that's the only time we could have conceived but had a negative test on the day of the bloody mucus therefore had no reason to think I was pregnant until today when I am 4 days late!! Has anyone else experienced this and when does the doctor calculate how many weeks you are from if the period was a fake ?? Also why could I have had a bleed if I had conceived?