blood test for pregnancy?

So on Friday night I had an extreme pain just above my right hip that hurt like crazy. I went to the doctors and my doctor made me take a pregnancy test which came up negative. I've just come off birth control which I've been on for about a year and a half. I'm on day 41 since my last period. My doctor has ordered me to take a bloojd test to see if I'm pregnant despite having a negative test, should I be concerned about being pregnant? I don't think I am. But I don't know why my doctor would order one if my test came up negative