Could I be pregnant even though test said No?

I'm 11 days late, and I never miss a cycle, I am tired, moody, got mive headaches, nauseous, and have heart burn. But no soreness or tenderness. Took a hpt 2 days ago and it was negative. I also have some minor cramping. There's no discharge of any kind and no signs of pms. Has anyone else ever had a test come out negative but was really positive? And how long did you wait to retest? I called the Ob but they can't see me for another week.