What Causes Delayed Menstruation With Headaches And Abdominal Bloating?

Hi my name is lindsay, I have been having cramps in my lower abdomin for the past two weeks, felt like I was getting my peiod, no period yet last cycle started on November 18th.. I am usually regular at 28-30 days, but I am working on my 5th day late. Just went to the ER and they said that the blood pregnancy test was negative but I have a yeast infection, though I do not have ANY symptoms of a yeast infection at all. My breasts still hurt, still cramping, feeling bloated frequent headaches.. The only expanation i can think of is that I am in fact pregnant but the ER did a qualitative blood test, and I think I may have low levels of HcG, please help