45 day since my last period blood work said negitive when taken on day 37

I have been off the will since April and since then I have been on a regular 28 day cycle. I am now 45 days since my last period on July 8 and i have done blood work, but it came back negative . I have been feeling very sleepy, and nauseous for the last few weeks and have very crampy. I am wondering if blood work can ever be wrong? Can i still possibly be pregnant ?