I have had sex with my boyfriend 20 days ago, and the date of my periods was 16th of the month but it has been 8 days above I am not getting my periods. I have checked my pregnancy test through a pregnancy strip but it show the negative result. I need to know what is the reason behind not getting my periods? Pregnancy?
And if I go for a blood test how can I detect weather I am pregnant or not?
Why am I not getting my periods though the result shows negative result?

You are saying your period is 8 days late? If so, it really is a bit early to test. Wait a week and test again. If it is still negative, you can wait another month, or see your doctor for a blood test. Some women never show positive results on urine tests, even when pregnant.