I have had this problem as well, and I read something like this....
Every pregnancy test looks for the pregnancy hormone in the blood or urine, and these hormones are only present when the woman is pregnant, this hormone is the human chronic gonadotropin (hcg). The urine and blood test look for the presence of hcg. The reason why your blood test might have shown negative is because there are two types of tests that the lab could have used to determine whether or not you are pregnant, one is the quantitative blood test and the other is the qualitative blood test. The qualitative hcg blood test is like the urine test as all it will give is a simple yes or no answer. A quantitative hcg blood test measures the exact amount of hcg in the blood and can pick up very small amounts of hcg, this makes it a very accurate test, and is much more accurate than the urine test and the qualitative blood test, so your blood test could have been the qualitative test and that is why it came back negative. What you need to do is another pregnancy test as you may indeed be pregnant, the spotting could be a implantation bleed that happens when the fertilized egg attaches itself to the lining of the uterus.
When are you due for a period again?