Tell me I'm not crazy...

I am 36 years young with 3 children (14, 12, and 3). My cycle is pretty consistent at 23 days (average). I am awaiting the results of a blood test as I am experiencing heightened sense of smell, random nausea, food aversions, constipation/diarrhea, and being 10 days late for my monthly visitor. So far all home/Dr office urine tests have been negative. I have never been this late without being pregnant. According to my LMP I would be 5weeks. Why wouldn't I show up pregnant on a urine test??