Yup I agree with Robyn... It could have been that when you went to take your Kaiser test there was not enough hormone in your urine. Testing positive in the morning, when you would have the highest concentration of hormone, and to test negative later that morning isn't surprising. The blood test will definately tell you for sure.
Also in my case, I spotted with all 3 of my kids, just like you had described. I was told that it was the egg implanting. But each woman is different obviously, as well as each pregnancy is different. I would definately see what happens with the blood test.
Good luck :)