A quick Google search unearthed 4-5 stories like this (mostly in response to women wondering why they have pregnancy symptoms but negative tests)!
Welllllll.... haha.. I got a negative blood test and SEVERAL negative HPT's... And.. I was definatly preggers... My daughter is 3.5 yrs now.. So.. Blood or urine... No test is ever 100%... (sorry).. hahaMakes you wonder.